Fix a type error in generics (#1913)

The function says it returns an `int`, but used `pass` so returned `None`. Fix by returning the `a` argument which is of type `int`.

Author: ndmitchell

docs/spec/generics.rst

@@ -1071,7 +1071,7 @@ outer ``Callable``.  This has the following semantics:
 .. code-block::
 
    def a_int_b_str(a: int, b: str) -> int:
-     pass
+     return a
 
    twice(a_int_b_str, 1, "A")       # Accepted