Fix type annotations in a documentation example

[3ok]

Changed the return type annotation for __init__ from int to None

Description

In one of the examples in mypyc's documentation, the constructor doesn't have a correct return type

class Point:
    def __init__(self, x: int, y: int) -> int:
        ...

I have changed it to None (according to mypy's documentation for example)

Test Plan

No particular test plan, it is a simple typo fix in the documentation.

[hauntsaninja]

Thanks!