Changed $task to $tag

getTags() returns collection of Tag objects therefore you want to add $tag not $task

Author: nixilla

cookbook/form/form_collections.rst

@@ -456,7 +456,7 @@ Next, add a ``by_reference`` option to the ``tags`` field and set it to ``false`
 
 With these two changes, when the form is submitted, each new ``Tag`` object
 is added to the ``Task`` class by calling the ``addTag`` method. Before this
-change, they were added internally by the form by calling ``$task->getTags()->add($task)``.
+change, they were added internally by the form by calling ``$task->getTags()->add($tag)``.
 That was just fine, but forcing the use of the "adder" method makes handling
 these new ``Tag`` objects easier (especially if you're using Doctrine, which
 we talk about next!).