Improve help message for the option "--resource-policies-configmap"

[draghuram]

Thank you for contributing to Velero!

Please add a summary of your change

Does your change fix a particular issue?

Fixes #(issue)

Please indicate you've done the following:

• [Yes ] Accepted the DCO. Commits without the DCO will delay acceptance.
• [ N/A] Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• [ N/A] Updated the corresponding documentation in site/content/docs/main.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.79%. Comparing base (33633d8) to head (7d61917).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7844 +/- ## ======================================= Coverage 58.79% 58.79% ======================================= Files 345 345 Lines 28764 28764 ======================================= Hits 16911 16911 Misses 10425 10425 Partials 1428 1428 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.