[gnome-shell-sass] Update links to use HEAD instead of master

[Florian Müllner <fmuellner src gnome org>]

commit d42044cd44427d3ca6d7151e780c61bc6480a175
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Jul 14 20:07:22 2021 +0200

    Update links to use HEAD instead of master
    
    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>

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index a5f219a..443b4dc 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,6 @@ will then be synchronized periodically before releases.
 GNOME Shell Sass is distributed under the terms of the GNU General Public
 License, version 2 or later. See the [COPYING][license] file for details.
 
-[shell-subtree]: https://gitlab.gnome.org/GNOME/gnome-shell/tree/master/data/theme/gnome-shell-sass
+[shell-subtree]: https://gitlab.gnome.org/GNOME/gnome-shell/tree/HEAD/data/theme/gnome-shell-sass
 [sass-repo]: https://gitlab.gnome.org/GNOME/gnome-shell-sass
 [license]: COPYING