users/contrib: Point to newer Syncthing-GTK fork (#873)

- https://github.com/kozec/syncthing-gtk is unmaintained.
- A fork of this project was made in
https://github.com/syncthing-gtk/syncthing-gtk
- Also it's the version available in Flatpak
https://flathub.org/apps/me.kozec.syncthingtk

---------

Co-authored-by: André Colomb <github.com@andre.colomb.de>

users/contrib.rst

@@ -273,7 +273,7 @@ Older, Possibly Unmaintained
    accordingly.
 
 - `a-sync <https://github.com/davide-imbriaco/a-sync>`_
-- `Syncthing-GTK <https://github.com/kozec/syncthing-gtk>`_
+- `Syncthing-GTK <https://github.com/syncthing-gtk/syncthing-gtk>`_ (Fork from `Kozec <https://github.com/kozec/syncthing-gtk>`_)
 -  https://github.com/syncthing/syncthing-lite
 -  https://github.com/sieren/QSyncthingTray
 -  https://github.com/akissa/pysyncthing