minor #10936 Update data_transformers.rst (sela)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #10936).

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Update data_transformers.rst

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4f8f825 Update data_transformers.rst

Author: javiereguiluz

form/data_transformers.rst

@@ -344,6 +344,7 @@ First, create the custom field type class::
     use AppBundle\Form\DataTransformer\IssueToNumberTransformer;
     use Doctrine\Common\Persistence\ObjectManager;
     use Symfony\Component\Form\AbstractType;
+    use Symfony\Component\Form\Extension\Core\Type\TextType;
     use Symfony\Component\Form\FormBuilderInterface;
     use Symfony\Component\OptionsResolver\OptionsResolver;