Changed reference to master branching in actions docs, should be main to match renaming

[anthonygrimes]

Why:

Closes #5341

What's being changed:

Changed reference to master branching in actions docs, should be main to match renaming.

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.

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[janiceilene]

Thanks so much for opening an issue and PR with this fix @anthonygrimes! This looks great ✨ I'm going to update the branch and get this merged in once tests are passing 🍏

edit: For some reason the "update branch" button is missing 🙃 so I'm troubleshooting that, then I'll get it merged in 😅

[janiceilene]

@anthonygrimes Can you enable the checkbox to allow maintainer edits? Then I'll be able to hit the update button and get this merged in. Thank you!

https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork

[anthonygrimes]

@janiceilene done!

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