Converted usage of ugettext* functions to their gettext* aliases

Thanks Tim Graham for the review.

docs/ref/models/instances.txt

@@ -261,7 +261,7 @@ access to more than a single field::
     import datetime
     from django.core.exceptions import ValidationError
     from django.db import models
-    from django.utils.translation import ugettext_lazy as _
+    from django.utils.translation import gettext_lazy as _
 
     class Article(models.Model):
         ...