Fixed query_builder option

Author: HeahDude

reference/forms/types/entity.rst

@@ -166,7 +166,7 @@ cast into a string and so must have a ``__toString()`` method.
 query_builder
 ~~~~~~~~~~~~~
 
-**type**: ``Doctrine\ORM\QueryBuilder`` or a Closure
+**type**: ``Doctrine\ORM\QueryBuilder`` or a Closure **default**: ``null``
 
 Allows you to create a custom query for your choices. See
 :ref:`ref-form-entity-query-builder` for an example.