fix deprecations: use fqcn for collectionType in getParent()

Form/Type/ChoicesType.php

@@ -3,22 +3,22 @@ namespace Chill\CustomFieldsBundle\Form\Type;
 
 use Symfony\Component\Form\AbstractType;
 use Symfony\Component\Form\FormBuilderInterface;
-
+use Symfony\Component\Form\Extension\Core\Type\CollectionType;
 
 /**
  *
  * @author Julien Fastré <julien.fastre@champs-libres.coop>
- *        
+ *
  */
 class ChoicesType extends AbstractType
-{    
+{
     public function getName()
     {
         return 'cf_choices';
     }
-    
+
     public function getParent()
     {
-        return 'collection';
+        return CollectionType::class;
     }
-}
+}