home (gnome): use caps lock as compose key

This key is present on my external keyboard and my laptop
keyboard which is not the case for right alt.

home/gnome/default.nix

@@ -21,7 +21,7 @@ in {
       };
       # set right alt as compose key for composing special letters suchs as umlauts
       "org/gnome/desktop/input-sources" = {
-        xkb-options = ["terminate:ctrl_alt_bksp" "lv3:ralt_switch" "compose:ralt"];
+        xkb-options = ["terminate:ctrl_alt_bksp" "lv3:ralt_switch" "compose:caps"];
       };
       "org/gnome/desktop/peripherals/touchpad" = {
         tap-to-click = true;