Fix type annotations in an example (#10351)

Changed the return type annotation for `__init__` from `int` to `None`

Author: 3ok

mypyc/doc/using_type_annotations.rst

@@ -83,7 +83,7 @@ Consider this example:
 .. code-block::
 
    class Point:
-       def __init__(self, x: int, y: int) -> int:
+       def __init__(self, x: int, y: int) -> None:
            self.x = x
            self.y = y