Fixes edge case where versions have prerelease info

[dmikusa]

Summary

Prior to this PR, if a version key in the index.yml had prerelease info, there would be an _ and then the prerelease info. This isn't valid semver, so it would break the pipeline. This change replaces the _ with a - which makes it valid semver.

Use Cases

Fixes broken runs like https://github.com/pivotal-cf/tanzu-appdynamics/runs/4732140660?check_suite_focus=true

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).