Change option label to match a translated one

Option label 'Any Date' is not included in any .po file but 'Any date' is in
django.contrib.admin. Changing this, language translation now works fine.

See [current django.contrib.admin django.po file](https://github.com/django/django/blob/master/django/contrib/admin/locale/en/LC_MESSAGES/django.po#L52).