Docs: Specify syntax for specifying version when using gitlab-ctl revert-pg-upgrade

What does this MR do?

In the docs, we tell people to use gitlab-ctl revert-pg-upgrade when providing guidance on how to revert packaged PostgreSQL server to a previous version.

• We offer the command as gitlab-ctl revert-pg-upgrade.
• We say that the command supports the -V flag to specify a target version.
• We do not provide information on how the -V flag is used.

This MR provides an example of how the -V flag is used to make it clear to folks following the docs what they should do.

This works to resolve the gitlab#364735 issue.

Deployed in a review app, the change looks like this:

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Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened

This is a documentation documentation-only change.