Converted usage of ugettext* functions to their gettext* aliases

Thanks Tim Graham for the review.

docs/topics/auth/passwords.txt

@@ -646,7 +646,7 @@ have a default value.
 Here's a basic example of a validator, with one optional setting::
 
     from django.core.exceptions import ValidationError
-    from django.utils.translation import ugettext as _
+    from django.utils.translation import gettext as _
 
     class MinimumLengthValidator(object):
         def __init__(self, min_length=8):