Change an incorrect word in the async/await docs (#2068)

This commit fixes a small error with the docs for async/await -- the
docs used the word "decorator" when it should have used the word
"generator".

Author: Michael0x2a

docs/source/kinds_of_types.rst

@@ -837,10 +837,9 @@ will be a value of type ``Awaitable[T]``.
 
    However, mypy currently does not support converting functions into
    coroutines. Support for this feature will be added in a future version, but
-   for now, you can manually force the function to be a decorator by doing
+   for now, you can manually force the function to be a generator by doing
    something like this:
 
-
    .. code-block:: python
    
       from typing import Generator