workspacesView: Fix off-by-one error

The scroll adjustment's upper value corresponds to the number of
workspaces, not to the last workspace index. We want the latter
when mirroring the layout in RTL locales, so subtract 1.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1598>

js/ui/workspacesView.js

@@ -300,7 +300,7 @@ class WorkspacesView extends WorkspacesViewBase {
         const vertical = workspaceManager.layout_rows === -1;
         const rtl = this.text_direction === Clutter.TextDirection.RTL;
         const progress = vertical || !rtl
-            ? adj.value : adj.upper - adj.value;
+            ? adj.value : adj.upper - adj.value - 1;
 
         for (const ws of this._workspaces) {
             if (vertical)