Correct the date format in the es_ES locale.

* es.po: Added a translation for the date format, because the default
does not show 4-digit years. This avoids the warning at startup, which
asks the translator to deal with it.
Note that I have made this correction before:
http://git.gnome.org/browse/glom/commit/po/es.po?id=79636cd099b8330339502cebe95fb6e1583d1c20