MODULAR PLASTIC FORM WORK SYSTEM FOR WALL TOP TECHNICAL FIELD The present invention relates to a formwork system used in building construction. BACKGROUND A formwork system may be composed of many different kinds of formworks, and it provides a framework for pouring of concrete for the main building. The conventional way of splicing the formworks is that: when a plane (such as, a side wall or a top wall) is spliced, it is realized by splicing of plane formworks; and when two mutually perpendicular planes (such as, two vertical walls intersected with each other) are spliced, it is realized by a combination of plane formworks and interior corner formworks. However, in a case of two mutually perpendicular side walls and a top wall (for example, as shown in FIG.1, a wall top body including two side walls 4 and a top wall 5), the three walls cannot be spliced together, by a conventional formwork system, because they are in three mutually perpendicular planes respectively. In prior art, the pouring of the wall top body is achieved by a way of compromise that building two vertical planes with plane formworks and interior corner formworks at first, pouring to have side walls, building a top plane with plane formworks, and plane formworks a top wall. However, because the pouring is split in two, there is likely to have an offset due to splicing, the quality of the surface is low after the pouring of concrete, and it is inefficient and takes long time and high cost. SUMMARY It is an object of the present invention to provide a modular plastic formwork system for wall top, with which the pouring of the wall top can be completed one-time. The technical solution to solve the technical problem of the present invention is as follow A modular plastic formwork system for wall top, having an X axis, a Y axis and a Z axis orthogonal to one another,. wherein the modular plastic formwork system comprises a formwork assembly for a surface of right angle and a formwork assembly for a top surface, the formwork assembly for a surface of right angle has two panels located on a plane of XZ and a plane of YZ respectively, and upper edges of the two panels form a right-angled corner, and the formwork assembly for a top surface comprises a panel located on a plane of XY with a right-angled edge which is fit closely and connected into one piece with the right-angled corner. As an improvement to the above technical solution, the formwork assembly for a surface of right angle comprises rectangular plate-like interior corner formworks and rectangular plate-like plane formworks; and at least one plane formwork is vertically arranged on each of the plane of XZ and the plane of YZ, and the interior corner formwork is used as an intermediate connector for connecting the plane formworks on the plane of XZ and on the plane of YZ. As an improvement to the above technical solution, the plane formwork is a flat rectangular plate-like plastic component having an obverse face and a reverse face, the obverse face is used as a panel, four sides of the reverse face are configured to form a rectangular framework therein, and the rectangular framework has long borders, short borders, and several plane connecting holes provided in the long borders and the short borders. As an improvement to the above technical solution, the interior corner formwork is a rectangular plate-like plastic component with an obverse face, a reverse face and four side faces, the obverse face of the interior corner formwork is used as a main panel of the interior corner formwork, the reverse face is provided with plane connecting holes, four sides of the reverse face are configured to form a rectangular framework with long borders and short borders, and each of the long border and the short border is provided with at least one side connecting hole; the long borders and short borders forms four side faces of the interior corner formwork; the rectangular framework is provided with a step in direction of a 2 short side of the rectangle; and a side face of the four side faces, which faces the long border, is used as a side panel. As an improvement to the above technical solution, the formwork assembly for a top surface comprises a three-side corner formwork, the three-side corner formwork is a rectangular plate component as a whole, and has a X direction connection face, a Y direction connection face and a Z direction connection face, wherein each of the X direction connection face and the Y direction connection face is connected to a short border of the interior corner formwork to form a right-angle edge of the formwork assembly for a top surface. As an improvement to the above technical solution, in the interior of the right-angle edge, there are several plane formworks, and the plane formwork is connected to an interior corner formwork of the formwork assembly for a top surface. As an improvement to the above technical solution, the three-side corner formwork is a rectangular plate-like plastic component with an obverse face, a reverse face and four side faces, the obverse face is used as a panel, the reverse face is used as a Z direction connection face, four sides of the reverse face are configured to form a rectangular framework provided with a high step and a low step, and the high step is arranged on one corner of the rectangular framework; two side faces against the high step are used as the X direction connection face and the Y direction connection face, the X direction connection face and the Y direction connection face are provided with a X direction connection hole and a Y direction connection hole respectively; and the reverse face is provided with a Z direction connection hole, and axes of the X direction connection hole, the Y direction connection hole and the Z direction connection hole are orthogonal to one another. The invention has the advantages that the modular plastic formwork system for wall top can build architecture of orthogonal formworks with three mutually perpendicular planes via the formwork assembly for a surface of right angle and the formwork assembly for a top surface, and due to the three mutually perpendicular planes, the architecture can solve the problem that the splicing of 3 the conventional formworks for wall top cannot be achieved one-time, to allow the pouring of the wall top being completed one-time, meanwhile, a more complicated wall top structure can be further spliced, based on the modular design of the present invention. In addition, configurations of the plane formwork, the interior corner formwork and the three-side corner formwork are designed to make the formwork system for wall top simple, convenient and easy to be assembled or disassembled. The present invention can be used widely in modern civil engineering construction. BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings. It was clear that the accompanying drawings are part of embodiments of the present invention, rather than all of embodiments. Other design schemes and accompanying drawings can be obtained for those skilled in the art based on the accompanying drawings, without creative work. FIG I shows a schematic view of a concrete wall top body. FIG 2 shows a schematic view of an overall assembled structure according to one embodiment of the present invention. FIG 3 shows an axonometric schematic view of a plane formwork with its obverse face upward according to one embodiment of the present invention. FIG 4 shows an axonometric schematic view of a plane formwork with its reverse face upward according to one embodiment of the present invention. FIG 5 shows an axonometric schematic view of a three-side corner formwork with its obverse face upward according to one embodiment of the present invention. FIG 6 shows an axonometric schematic view of a three-side corner formwork with its reverse face upward according to one embodiment of the present invention. 4 FIG 7 shows an axonometric schematic view of an interior corner formwork with its obverse face upward according to one embodiment of the present invention. FIG 8 shows an axonometric schematic view of an interior corner formwork with its reverse face upward according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION In the following description of embodiments, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific embodiments of the present invention that can be practiced. It is to be understood that other embodiments can be used and structural changes can be made without departing from the scope of the present invention. In addition, all of join relationships or connection relationships mentioned herein do not only a mean direct connection between components, but an appropriate connection structure formed by adding or reducing a connection accessory based on the actual application. Referring to FIG 2, a modular plastic formwork system for wall top has an X axis, a Y axis and a Z axis orthogonal to one another, the modular plastic formwork system includes a formwork assembly for a surface of right angle and a formwork assembly for a top surface, the formwork assembly for a surface of right angle has two panels located on a plane of XZ and a plane of YZ, and upper edges of the two panels form a right-angled corner; and the formwork assembly for a top surface includes a panel located on a plane of XY with a right-angled edge, and the right-angled edge of the plane is fit closely and connected with the right-angled corner. As a preferred embodiment, the formwork assembly for a surface of right angle includes rectangular plate-like interior corner formworks 2 and plane formworks 1. At least one plane formwork I is arranged on each of the plane of XZ and the plane of YZ in vertical direction, and the interior corner formwork 2 is used as an intermediate connector to connect with the plane formworks I on the 5 plane of XZ and the plane of YZ. Referring to FIGS. 3 and 4, as a preferred embodiment, the plane formwork I is a flat rectangular plate-like plastic component with an obverse face and a reverse face, the obverse face is used as a panel, four sides of the reverse face are configured to form a rectangular framework, and the rectangular framework has long borders I a, short borders I b, and several plane connecting holes I c provided in the long borders I a and the short borders lb. Specifically, at least one annular reinforcing rib is provided in the rectangular framework of the plane formwork I to reinforce solidity. The thickness, width, length or other parameters of the plane formwork I can be adjusted according to the different needs for various occasions. Referring to FIGS 7 and 8, as a preferred embodiment, the interior corner formwork 2 is a rectangular plate-like plastic component with an obverse face, a reverse face and four side faces, the obverse face of the interior corner formwork 2 is used as a main panel of the interior corner formwork, the reverse face is provided with plane connecting holes 2c, four sides of the reverse face are configured to form a rectangular framework with long borders 2a and short borders 2b, and the long border 2a and short border 2b are provided with one side connecting hole 2d respectively. The long borders 2a and short borders 2b forms four side faces of the interior corner formwork 2. The rectangular framework is provided with a step in direction of a short side of a rectangle. A side face of the four side faces, which faces the long border, is used as a side panel. Specifically, at least one annular reinforcing rib is provided in the rectangular framework of the interior corner formwork 2 to reinforce solidity. Referring to FIGS. 5 and 6, as a preferred embodiment, the formwork assembly for a top surface includes a three-side corner formwork 3, the three-side corner formwork 3 is a rectangular plate component as a whole, with a X direction connection face 3a, a Y direction connection face 3b and a Z direction connection face 3c, and the X direction connection face 3a and the Y direction connection face 3b are connected to a short border 2b of the interior corner formwork 2 respectively to form right-angle edges of the formwork assembly for a top surface. As a preferred embodiment, the right-angle edge is provided with several 6 plane formworks I inside, and the plane formwork I is connected to an interior corner formwork 2 of the formwork assembly for a top surface. As a preferred embodiment, the three-side corner formwork 3 is a rectangular plate-like plastic component with an obverse face, a reverse face and four side faces, the obverse face is used as a panel, the reverse face is used as Z direction connection face 3c, four sides of the reverse face are configured to form a rectangular framework provided with a high step and a low step, and the high step is arranged on one corner of the rectangular framework. Two side faces against the high step are used as the X direction connection face 3a and the Y direction connection face 3b, the X direction connection face 3a and the Y direction connection face 3b are provided with a X direction connection hole 3d and a Y direction connection hole 3e respectively. The reverse face is provided with a Z direction connection hole 3f, and axes of the X direction connection hole 3d, the Y direction connection hole 3e and the Z direction connection hole 3f are orthogonal to one another. I-low the present invention is assembled is detailed below. First, two plane formworks I are taken, arranged in the same direction that their long borders la are combined, and connected together via connecting bolts through the plane connecting holes le to form a panel assembly with the plane connecting holes Ic around. Three panel assemblies are prepared. Then, at least two interior corner formworks 2 are taken, arranged to make their main panel face in the same direction that their short borders 2b are combined, and connected together via connecting bolts through the side connecting holes 2d to form an axle body. Three axle bodies are prepared. And then, a three-side corner formwork 3 is prepared and placed horizontally, to make the X direction connection face 3a, X direction connection face 3b and Z direction connection face 3c face the X direction, Y direction, and Z direction respectively. The short borders 2b of the three axle bodies which have already been made are arranged to press against the X direction connection face 3a, X direction connection face 3b and Z direction connection face 3c respectively, and all of the outward planes are flush with one another. The three axle bodies and the three-side corner formwork 3 are connected together with the X direction connection hole 3d, the Y direction connection hole 3e, the Z direction connection hole 3f and the side connection hole 2d, to make a three-axis framework. Finally, the three panel assemblies are placed horizontally, verticality in front and verticality in side respectively, to make the plane connecting holes le of the panel assembly located on the top be aligned to the side connection hole 2d of the upper part of the three-side corner formwork, and the connecting bolts are tightened. The two panel assemblies located on the bottom can constitute a right-angle relationship with the vertical axle body of the three-axis framework (i.e., interior corner formwork 2 connected in series in the vertical direction) to form two orthogonal planes. In this way, all of the plane connecting holes Ic on the sides of one panel assembly can be aligned to connecting holes of other formworks, such as plane connecting holes 2c and Z direction connection hole 3f, and a formwork system for wall top as shown in FIG 2 is built by tightening the connecting bolt. It should be noted that the connection relationships between the parts of the system are described for illustration, and it does not mean that the system can only be assembled by the above steps. In practice, the parts of the system should be selected, arranged and assembled according to the condition in the construction. The plane formwork 1, interior corner formwork 2 and three-side corner formwork 3 of the system can be freely combined and used. For example, when an interior wall is spliced, the plane formwork I and the interior corner formwork 2 can be combined and spliced, and when a ceiling is spliced, the plane formwork 1, the interior corner formwork 2 and the three-side corner formwork 3 can be combined and spliced. The described embodiments of the present invention are presents for the purpose of illustration, not limitation. Those skilled in the art will recognize that the present invention can be practiced with modification within the spirit and scope of the claims. 8