Change 'master' branch labels to 'main'

[Two4]

Why:

Following the guide leads a confusing extra non-default 'master' branch on new repositories

What's being changed:

instructions lead to files being pushed to 'main' instead of 'master' on Windows/Linux (Mac already changed as of this PR)

Check off the following:

• [x ] All of the tests are passing.
• [ x] I have reviewed my changes in staging.
• [x ] For content changes, I have reviewed the localization checklist
• [x ] For content changes, I have reviewed the Content style guide for GitHub Docs.
  I've actually done none of these, it's literally a two word change. MWAHAHAHA

[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]

Thanks so much for this PR @Two4! I'll update this branch and merge once all the tests are green 🍏

[janiceilene]

If you haven't already, you can add yourself to the list of contributors by creating a new comment in this PR using these instructions. Thanks again! ✨