docs: Fix branch prefix detection with dashes

Created by: LawnGnome

I noticed this when admiring the draft campaign documentation in #10921 — if you go to a link that was rewritten to maintain the version prefix, such as any of the links in the sidebar screenshot below, the prefix is erroneously @campaigns instead of @campaigns-new-flow:

There's probably an argument for this to just match anything except /, but I've chosen to be conservative here.