Fix ValueError in scenario when creating dunning procedure

The problem was introduced in changeset 15f4b2bf9765
issue5718
review33311003

tests/scenario_account_dunning_letter.rst

@@ -91,13 +91,13 @@ Create dunning procedure::
     >>> procedure = Procedure(name='Procedure')
     >>> level = procedure.levels.new()
     >>> level.sequence = 1
-    >>> level.days = datetime.timedelta(5)
+    >>> level.overdue = datetime.timedelta(5)
     >>> level = procedure.levels.new()
     >>> level.sequence = 2
-    >>> level.days = datetime.timedelta(20)
+    >>> level.overdue = datetime.timedelta(20)
     >>> level = procedure.levels.new()
     >>> level.sequence = 3
-    >>> level.days = datetime.timedelta(40)
+    >>> level.overdue = datetime.timedelta(40)
     >>> procedure.save()
 
 Create parties::