fix(documentation): Use right flags for export command

[rmarting]

The current export command text is not aligned with the real flags used by the command.

The command requires to use output-file however the help sais to use file flag.

Verification Steps

1. Display the export help: `rhoas service-registry artifact export --help'
2. Execute the export: rhoas service-registry artifact export --output-file=export.zip

Type of change

• [] Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation change
• [] Other (please specify)

[rkpattnaik780]

Thanks a lot for this PR @rmarting :).