FormBuilderInterface: fix getForm() return type.

FormBuilderInterface->getForm() should depend on abstractions and
not implementations as a return type.

src/Symfony/Component/Form/FormBuilderInterface.php

@@ -81,7 +81,7 @@ interface FormBuilderInterface extends \Traversable, \Countable, FormConfigBuild
     /**
      * Creates the form.
      *
-     * @return Form The form
+     * @return FormInterface The form
      */
     public function getForm();
 }