keyboard: Fix fallback layout when using variants

Commit c1ec7b2ff meant to fall back to the base layout in case
a variant like `fr+oss` is set up, but as we are checking for
'+' on the array rather than the layout name, the fallback only
"works" for a layout that is literally called '+', whoops.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2471

js/ui/keyboard.js

@@ -498,7 +498,7 @@ var Key = GObject.registerClass({
 var KeyboardModel = class {
     constructor(groupName) {
         let names = [groupName];
-        if (names.includes('+'))
+        if (groupName.includes('+'))
             names.push(groupName.replace(/\+.*/, ''));
         names.push('us');