Converted usage of ugettext* functions to their gettext* aliases

Thanks Tim Graham for the review.

docs/howto/custom-model-fields.txt

@@ -498,7 +498,7 @@ instances::
 
     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 _
 
     def parse_hand(hand_string):
         """Takes a string of cards and splits into a full hand."""