Iterate on the result of items call to update values set by on_change on O2M

e38f2298 introduces wrongly the iteration over the function instead of the result.

Closes #14014
(grafted from fadb43303291d5641f5e30169126e595aaa83ad5)

--HG--
branch : 6.0

tryton/tryton/gui/window/view_form/model/field.py

@@ -827,7 +827,7 @@ class O2MField(Field):
                 record2 = group.get(vals['id'])
                 if record2 is not None:
                     to_set = {
-                        k: v for k, v in vals.items
+                        k: v for k, v in vals.items()
                         if k not in vals_to_set}
                     record2.set_on_change(to_set)