minor #5466 data transformers cookbook service definition typo (intrepion)

This PR was merged into the 2.3 branch.

Discussion
----------

data transformers cookbook service definition typo

Commits
-------

b3414de data transformers cookbook service definition typo

Author: xabbuh

cookbook/form/data_transformers.rst

@@ -149,7 +149,7 @@ to explicitly choose the one to use. To achieve this, you can use a factory::
             app.type.task:
                 class: AppBundle\Form\TaskType
                 arguments: ["@app.issue_transformer_factory"]
-                tag:
+                tags:
                   - { name: form.type, alias: app_task }
 
     .. code-block:: xml