Corrected the default visibility timeout (#4476)

According to kombu, the default visibility timeout is 30 minutes.

https://github.com/celery/kombu/blob/3a7cdb07c9bf75b54282274d711af15ca6ad5d9f/kombu/transport/SQS.py#L85

docs/getting-started/brokers/sqs.rst

@@ -76,7 +76,7 @@ This option is set via the :setting:`broker_transport_options` setting::
 
     broker_transport_options = {'visibility_timeout': 3600}  # 1 hour.
 
-The default visibility timeout is 30 seconds.
+The default visibility timeout is 30 minutes.
 
 Polling Interval
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