fix return type with multiple in form

Author: MrYamous

reference/forms/types/options/multiple.rst.inc

@@ -6,4 +6,5 @@
 If true, the user will be able to select multiple options (as opposed
 to choosing just one option). Depending on the value of the ``expanded``
 option, this will render either a select tag or checkboxes if true and
-a select tag or radio buttons if false. The returned value will be an array.
+a select tag or radio buttons if false. The returned value will be a
+Doctrine's Array Collection.