Update updating-credentials-from-the-macos-keychain.md

[victorlin]

Cloning public repositories does not prompt for credentials.

Why:

Current documentation is not accurate.

What's being changed:

Clarify that cloning only private repositories will prompt authentication.

• current: https://docs.github.com/en/github/using-git/updating-credentials-from-the-macos-keychain
• staging: https://docs-3610--patch-1.herokuapp.com/en/github/using-git/updating-credentials-from-the-macos-keychain

Check off the following:

• 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.

[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.

[janiceilene]

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

[myarb]

Thank you so much for making this change! I'm updating this branch from main and we'll get it merged 🔜