[AIRFLOW-6509] Description for how to create an user is incorrect in some docs.

[sarutak]

In start.rst and security.rst, description for how to create user is wrong.
We can't do like

airflow users -c ...

Instead, we should do like
``
airflow users create ...

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Issue link: [AIRFLOW-6509](https://issues.apache.org/jira/browse/AIRFLOW-6509/)

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