typing docs: Add example for async functions (GH-20386)

Fixes python/typingGH-424
(cherry picked from commit 9588f88)

Co-authored-by: Sam Bull <aa6bs0@sambull.org>

Author: miss-islington

Doc/library/typing.rst

@@ -194,6 +194,10 @@ For example::
                    on_error: Callable[[int, Exception], None]) -> None:
        # Body
 
+   async def on_update(value: str) -> None:
+       # Body
+   callback: Callable[[str], Awaitable[None]] = on_update
+
 It is possible to declare the return type of a callable without specifying
 the call signature by substituting a literal ellipsis
 for the list of arguments in the type hint: ``Callable[..., ReturnType]``.