Update fields_default_value doc to Active Record

doc/topics/models/fields_default_value.rst

@@ -7,7 +7,7 @@ Default value of fields
 When a record is created, each field, which doesn't have a value specified,
 is set with the default value if exists.
 
-The following method::
+The following class method::
 
     Model.default_<field name>()
 
@@ -21,14 +21,11 @@ This example defines an ``Item`` model which has a default ``since``::
 
     class Item(ModelSQL, ModelView):
         "Item"
-        _description = __doc__
-        _name = "item.item"
+        __name__ = 'item'
         since = fields.Date('since')
 
-        def default_since(self):
+        @classmethod
+        def default_since(cls):
             return datetime.date.today()
 
-    Item()
-
 See also method ``Model.default_get``: :attr:`~trytond.model.Model.default_get`
-