fix typo in generate_mask.py example usage

[mohseniaref]

typo , I just added -o

Description of proposed changes

Reminders

• Fix #xxxx
• Pass Pre-commit check (green)
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• Make sure that your code follows our style. Use the other functions/files as a basis.
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• If adding new functionality, add a detailed description to the documentation and/or an example.

[yunjunz]

Thank you @mohseniaref.