Capitalization of "Postal Code" is inconsistent

…with Drupal standards, and also with where it appears elsewhere in the Address Field codebase. Specifically, I'm speaking of the title of the field as it appears in plugins/format/address.inc, and therefore on the field title on actual forms which use the standard postal code field. (This is also causing translators to have to translate both "Postal code" and "Postal Code.")