minor #6166 Fix by_reference deprecated FormType::class (nemo-)

This PR was merged into the 2.8 branch.

Discussion
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Fix by_reference deprecated FormType::class

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    |  2.8, 3.0, 3.1
| Fixed tickets | -

Commits
-------

5d1806e Fix by_reference deprecated FormType::class

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

@@ -12,13 +12,14 @@ To explain this further, here's a simple example::
 
     use Symfony\Component\Form\Extension\Core\Type\TextType;
     use Symfony\Component\Form\Extension\Core\Type\EmailType;
+    use Symfony\Component\Form\Extension\Core\Type\FormType;
     // ...
 
     $builder = $this->createFormBuilder($article);
     $builder
         ->add('title', TextType::class)
         ->add(
-            $builder->create('author', 'form', array('by_reference' => ?))
+            $builder->create('author', FormType::class, array('by_reference' => ?))
                 ->add('name', TextType::class)
                 ->add('email', EmailType::class)
         )