[gnome-shell-extensions] docs: Use HEAD in external URLs

[Marge Bot <marge-bot src gnome org>]

commit a642c439ce8d6a3627895f5e250cf9f44fa0ff80
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Jul 14 20:37:15 2021 +0200

    docs: Use HEAD in external URLs
    
    That way the link will keep working when the other project changes
    its default branch name.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/177>

 HACKING.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/HACKING.md b/HACKING.md
index f878628..6d87c52 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -28,4 +28,4 @@ imports (like imports.lang or imports.dbus) and introspection,
 the other for Shell API. Within the same group, put everything
 in alphabetic order.
 
-[coding-style]: https://gitlab.gnome.org/GNOME/gjs/blob/master/doc/Style_Guide.md
+[coding-style]: https://gitlab.gnome.org/GNOME/gjs/blob/HEAD/doc/Style_Guide.md