doc: update phrasing to use default branch rather than main in Syncing a Fork

[bnb]

Why:

When I was less experienced, I absolutely would have gotten confused by usage of main in a repo where main isn't the default branch. Since GitHub has landed on default branch as the phrasing for the concept, I've gone ahead and updated this to specifically call out default branch in a way that still uses main as an example.

What's being changed:

Prose around main, giving more context that main is just the default branch for this specific example.

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging.
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[janiceilene]

Thanks for opening a PR @bnb! I'll get this triaged for review ⚡

[hubwriter]

@bnb - This is a good change. Many thanks for submitting this. We really appreciate your help improving GitHub documentation.

We'll get this change merged in very soon.

[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.

[bnb]

Thanks, y'all! ❤️