api-docs: Update narrow usage examples for get_messages.

Updates narrow parameters in the javascript and python usage
examples for the get_messages endpoint to use the "channel"
operator instead of the "stream" operator.

Part of the stream to channel rename project.

zerver/openapi/python_examples.py

@@ -967,7 +967,7 @@ def get_messages(client: Client) -> None:
         "num_after": 0,
         "narrow": [
             {"operator": "sender", "operand": "iago@zulip.com"},
-            {"operator": "stream", "operand": "Verona"},
+            {"operator": "channel", "operand": "Verona"},
         ],
     }
     result = client.get_messages(request)