[example] Fix bad example (#32161)

Channel object has no method ``stop``, it supposed to be ``close`` here.

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Co-authored-by: Xuan Wang <xuanwn@google.com>

examples/python/multiprocessing/client.py

@@ -41,9 +41,9 @@ _LOGGER = logging.getLogger(__name__)
 
 
 def _shutdown_worker():
-    _LOGGER.info('Shutting worker process down.')
+    _LOGGER.info("Shutting worker process down.")
     if _worker_channel_singleton is not None:
-        _worker_channel_singleton.stop()
+        _worker_channel_singleton.close()
 
 
 def _initialize_worker(server_address):