Update detaching-a-fork.md

[jkopczyn]

Clarify what to do with an error message about refusing to delete branches.

Why:

Got a somewhat scary message when the repository was old enough to use master and the new copy used main. Add a warning about this for future reference.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Deleting a fork instructions; add a warning.

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
pull-requests/collaborating-with-pull-requests/working-with-forks/detaching-a-fork.md	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

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

[jkopczyn]

I'm unclear on the linter's objection so I'll leave this for someone more familiar with the style guidelines to fix.

[nguyenalex836]

@jkopczyn Let me see if just updating the branch resolves 🤞 if not, one of our other team members can assist with the linter error upon reviewing this contribution 💛