fix the path to the file in the example code

Author: gimler

cookbook/form/data_transformers.rst

@@ -21,7 +21,7 @@ a text field, as we configure the fields' parent to be a "text" field, in which
 will enter the issue number. The field will display an error if a non existing number
 was entered::
 
-    // src/Acme/TaskBundle/Form/IssueSelectorType.php
+    // src/Acme/TaskBundle/Form/Type/IssueSelectorType.php
     namespace Acme\TaskBundle\Form\Type;
 
     use Symfony\Component\Form\AbstractType;