Use ::class instead of some hardcoded class names

This gives us better refactoring functionality in IDEs like
PhpStorm, and also more quickly surfaces typos through errors
about undefined classes. :)

src/Api/Controller/ListGroupsController.php

@@ -11,6 +11,7 @@
 
 namespace Flarum\Api\Controller;
 
+use Flarum\Api\Serializer\GroupSerializer;
 use Flarum\Group\Group;
 use Psr\Http\Message\ServerRequestInterface;
 use Tobscure\JsonApi\Document;
@@ -20,7 +21,7 @@ class ListGroupsController extends AbstractListController
     /**
      * {@inheritdoc}
      */
-    public $serializer = 'Flarum\Api\Serializer\GroupSerializer';
+    public $serializer = GroupSerializer::class;
 
     /**
      * {@inheritdoc}