minor symfony#10780 Complete code example for "Adding validation" part (welcoMattic)

This PR was squashed before being merged into the 3.4 branch (closes symfony#10780).

Discussion
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Complete code example for "Adding validation" part

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This fix will avoid confusion for beginners

Commits
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6abfc12 Complete code example for \"Adding validation\" part

Author: javiereguiluz

form/without_class.rst

@@ -82,18 +82,22 @@ but here's a short example::
     use Symfony\Component\Validator\Constraints\Length;
     use Symfony\Component\Validator\Constraints\NotBlank;
     use Symfony\Component\Form\Extension\Core\Type\TextType;
+    use Symfony\Component\Form\FormBuilderInterface;
 
-    $builder
-       ->add('firstName', TextType::class, array(
-           'constraints' => new Length(array('min' => 3)),
-       ))
-       ->add('lastName', TextType::class, array(
-           'constraints' => array(
-               new NotBlank(),
-               new Length(array('min' => 3)),
-           ),
-       ))
-    ;
+    public function buildForm(FormBuilderInterface $builder, array $options)
+    {
+        $builder
+           ->add('firstName', TextType::class, array(
+               'constraints' => new Length(array('min' => 3)),
+           ))
+           ->add('lastName', TextType::class, array(
+               'constraints' => array(
+                   new NotBlank(),
+                   new Length(array('min' => 3)),
+               ),
+           ))
+        ;
+    }
 
 .. tip::