[Aio] Fix typo in async graceful shutdown example (#29521)

Background: https://github.com/grpc/grpc/pull/26622

examples/python/helloworld/async_greeter_server_with_graceful_shutdown.py

@@ -45,7 +45,7 @@ async def serve() -> None:
 
     async def server_graceful_shutdown():
         logging.info("Starting graceful shutdown...")
-        # Shuts down the server with 0 seconds of grace period. During the
+        # Shuts down the server with 5 seconds of grace period. During the
         # grace period, the server won't accept new connections and allow
         # existing RPCs to continue within the grace period.
         await server.stop(5)