ref_name-description.md: fix format for PRs

[aspiers]

Why:

The current docs for github.ref_name do not represent the reality of how it behaves. Instead, they claim that for PRs, the format is refs/pull/<pr_number>/merge. This claim comes from PR #29712 which was submitted to address issue #15319.

However when referring to github.ref and github.ref_name, #15319 states:

Instead these variables are populated by values like refs/pull/26/merge and 26/merge respectively.

I have just tested this and found that the actual format is indeed <pr_number>/merge, not the longer version. (This makes sense, since it's more consistent with github.ref_name being the short form of github.ref in other cases.)

What's being changed (if available, include any code snippets, screenshots, or gifs):

This PR fixes the docs to represent the actual reality that github.ref_name always returns the short format, even for PRs.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline.

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[nguyenalex836]

@aspiers Thanks so much for opening a PR! I'll get this triaged for review ✨

[hubwriter]

@aspiers - Many thanks for checking and correcting this.

Much appreciated. 👍

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues ⚡