[mutter] docs: Update coding style link

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

commit 2c4db093552bd04ee7823d4332fcc95996adf4e3
Author: Sebastian Keller <skeller gnome org>
Date:   Tue Nov 9 19:44:37 2021 +0100

    docs: Update coding style link
    
    The old one seems to have stopped working at some point due to all the
    recent changes to the developer website.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2084>

 HACKING.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/HACKING.md b/HACKING.md
index 5c3a9945f8..ec65002f65 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -283,4 +283,4 @@ click_action_clicked_cb (ClutterClickAction *click_action,
 }
 ```
 
-[gnome-coding-style]: https://developer.gnome.org/programming-guidelines/stable/c-coding-style.html.en
+[gnome-coding-style]: https://developer.gnome.org/documentation/guidelines/programming/coding-style.html