Converted usage of ugettext* functions to their gettext* aliases

Thanks Tim Graham for the review.

docs/ref/contrib/admin/index.txt

@@ -860,7 +860,7 @@ subclass::
            from datetime import date
 
            from django.contrib import admin
-           from django.utils.translation import ugettext_lazy as _
+           from django.utils.translation import gettext_lazy as _
 
            class DecadeBornListFilter(admin.SimpleListFilter):
                # Human-readable title which will be displayed in the