Fixed typo with asynccontextmanager code example (pythonGH-8845)

`yield conn`, instead of just `yield`.
(cherry picked from commit 416cbce)

Co-authored-by: Alexander Vasin <hi@alvass.in>

Author: alvassin

Doc/library/contextlib.rst

@@ -116,7 +116,7 @@ Functions and classes provided:
       async def get_connection():
           conn = await acquire_db_connection()
           try:
-              yield
+              yield conn
           finally:
               await release_db_connection(conn)