Docs: Fix RegexError reference
gi-docgen
version 2022.2 throws an error on this reference,
but even if it's accepted it ends up as a 404 link. (See, for example, the current live docs, where that GRegexError
reference is a bad link.
Using [error@
correctly links to https://docs.gtk.org/glib/error.RegexError.html in the generated documentation.