gd_GB: Fix typo in abbreviated "May" (bug 23152).

	[BZ #23152]
	* localedata/locales/gd_GB (abmon): Fix typo in May:
	"Mhàrt" -> "Cèit".  Adjust the comment according to the change.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit bb066cb8)