Fix GITHUB_{HEAD|BASE}_REF env var descriptions

[lucascosti]

Why:

Closes: #673

The description of the GITHUB_HEAD_REF and GITHUB_BASE_REF GitHub Actions environment variables are incorrect. I have confirmed with engineering that these are set for every pull request event, and not just forks.

What's being changed:

Fixed the descriptions to state that they apply to all PR events.

Staged preview: https://docs-1955--lucascostiactions-p.herokuapp.com/en/free-pro-team@latest/actions/reference/environment-variables#default-environment-variables

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[lucascosti]

closing and reopening to try to fix the heroku deploy....

[martin389]

Looks good to me, thanks @lucascosti 🎸

[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.