That way the link will keep working when projects change their default branch name. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1914>
After the move to Gitlab, the gnome-shell-sass repository is more exposed than it used to; clarify that it is not the place where style changes should happen, but rather the corresponding subtree in the gnome-shell repository.
Markdown produces nicer output where supported, so use that instead of plain ASCII formatting.