Fixed the option definition

Author: javiereguiluz

reference/constraints/UniqueEntity.rst

@@ -256,7 +256,7 @@ each with a single field.
 ``ignoreNull``
 ~~~~~~~~~~~~~~
 
-**type**: ``bool``, ``string`` or ``array`` **default**: ``[]``
+**type**: ``bool`` | ``string`` | ``array`` **default**: ``true``
 
 If this option is set to ``true``, then the constraint will allow multiple
 entities to have a ``null`` value for a field without failing validation.