Changing class attribute to array from string.

Closes #3514936

core/modules/system/src/Form/ModulesUninstallForm.php

@@ -174,7 +174,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
             Url::fromUri($module->info[ExtensionLifecycle::LIFECYCLE_LINK_IDENTIFIER], [
               'attributes' =>
                 [
-                  'class' => 'module-link--non-stable',
+                  'class' => ['module-link--non-stable'],
                   'aria-label' => $this->t('View information on the @lifecycle status of the module @module', [
                     '@lifecycle' => ucfirst($lifecycle),
                     '@module' => $module->info['name'],