[1.7.x] Fixed #22622 -- Added ugettext_lazy import in docs

Backport of 7271f5db78 from master

docs/topics/forms/modelforms.txt

@@ -513,6 +513,8 @@ attributes of the inner ``Meta`` class if you want to further customize a field.
 For example if you wanted to customize the wording of all user facing strings for
 the ``name`` field::
 
+    from django.utils.translation import ugettext_lazy as _
+
     class AuthorForm(ModelForm):
         class Meta:
             model = Author