Update choice_loader.rst.inc

Hello, 
A small suggestion: Isn't it better to extend the class `ConstantsType` from the `Form\AbstractTypeExtension` instead? As we're using the function `getExtendedTypes` i think it might be better. This method is available in the `AbstractTypeExtension`  class and not in the `AbstractType` class. I might be wrong but here we're about to create a type extension from the ChoiceType.

Author: issamkhadiri1989

reference/forms/types/options/choice_loader.rst.inc

@@ -34,13 +34,13 @@ better performance::
 
     use App\Form\ChoiceList\CustomChoiceLoader;
     use App\StaticClass;
-    use Symfony\Component\Form\AbstractType;
+    use Symfony\Component\Form\AbstractTypeExtension;
     use Symfony\Component\Form\ChoiceList\ChoiceList;
     use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
     use Symfony\Component\OptionsResolver\Options;
     use Symfony\Component\OptionsResolver\OptionsResolver;
 
-    class ConstantsType extends AbstractType
+    class ConstantsType extends AbstractTypeExtension
     {
         public static function getExtendedTypes(): iterable
         {