Fix migrate SQL command message regarding config file.

[dkushner]

Related issue

#1411

Proposed changes

Simple changes the help message output when a user tries to invoke the migrate sql subcommand without specifying a database URL on the command line itself. Currently the message for -e indicates that it will pull from the DSN environment variable but not that it will check the dsn key in the indicated configuration file.

Checklist

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• I have added tests that prove my fix is effective or that my feature works
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• I have documented my changes in the developer guide (if appropriate)

[aeneasr]

Thank you!