refactor: change default name for model format to improve help text

[ewanharris]

Description

Changes the default format to autodetect to better describe the default behaviour

fga model transform help text

Flags:
      --file fga.mod           File Name. The file should have the model in the JSON or DSL format or be an fga.mod format
  -h, --help                   help for transform
      --input-format format    Authorization model input format. Can be "fga", "json", or "modular" (default autodetect)
      --output-format format   Authorization model output format. Can be "fga" or "json"." (default autodetect)	

References

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected