Fixed error when saving custom design modal

no issue

- `Promise.all()` requires a promise so it was erroring about uniterable symbols

ghost/admin/app/controllers/settings/design/customize.js

@@ -15,10 +15,10 @@ export default class SettingsDesignCustomizeController extends Controller {
                 return;
             }
 
-            yield Promise.all(
+            yield Promise.all([
                 this.settings.save(),
                 this.customThemeSettings.save()
-            );
+            ]);
 
             this.router.transitionTo('settings.design');