Fix Sphinx error in CLI reference docs for dags test command

[roykoand]

The CLI reference docs page is generated from the argparse help strings, and the dags test description was not valid reStructuredText: without a blank line before the numbered list, docutils reported "Unexpected indentation" and Sphinx rendered a System Message error box instead of the usage examples on cli-and-env-variables-ref.html. Verify by checking this URL - https://airflow.apache.org/docs/apache-airflow/stable/cli-and-env-variables-ref.html#test_repeat1

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