Fixed typo with asynccontextmanager code example (GH-8845)


`yield conn`, instead of just `yield`.
(cherry picked from commit 416cbce22d5d8879986125f07b3e293421bc975d)

Co-authored-by: Alexander Vasin <hi@alvass.in>
diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst
index 793bd63..7dc5b29 100644
--- a/Doc/library/contextlib.rst
+++ b/Doc/library/contextlib.rst
@@ -116,7 +116,7 @@
       async def get_connection():
           conn = await acquire_db_connection()
           try:
-              yield
+              yield conn
           finally:
               await release_db_connection(conn)