workspace-indicator: Use consistent workspace numbering

The indicator numbers workspaces starting from 1, while newly added
workspace names in the preference dialog start counting at 0.
Change the latter to be consistent with the indicator.

https://bugzilla.gnome.org/show_bug.cgi?id=753105

extensions/workspace-indicator/prefs.js

@@ -195,7 +195,7 @@ const WorkspaceSettingsWidget = new GObject.Class({
         let iter = this._store.append();
         let index = this._store.get_path(iter).get_indices()[0];
 
-        let label = _("Workspace %d").format(index);
+        let label = _("Workspace %d").format(index + 1);
         this._store.set(iter, [this._store.Columns.LABEL], [label]);
     },