Fix upgrade docs to reflect true cli flags available

[jdistler]

The upgrade docs section concerning Offline SQL migration scripts are incorrect and reference cli flags which are not supported by the latest release of airflow -

The flags -r and --revision-range are dead code with no references (to be cleaned up in a subsequent PR). These flags are not used by the airflow db migrate command. The true set of flags are these.

I have updated the documentation to reflect that users should actually be using the flags --show-sql-only and --from-version.

[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.