[ServiceBus] Sample async await (Azure#24858)

* ce await async

* uncomment

Author: l0lawrence

sdk/servicebus/azure-servicebus/samples/async_samples/connection_to_custom_endpoint_address_async.py

@@ -28,7 +28,7 @@
 
 async def send_single_message(sender):
     message = ServiceBusMessage("Single Message")
-    sender.send_messages(message)
+    await sender.send_messages(message)
 
 async def main():
     servicebus_client = ServiceBusClient.from_connection_string(