tzsetup: Correct UTC description

UTC is Coordinated Universal Time, not Greenwich Mean Time.

Reviewed by: imp, allanjude
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45573

(cherry picked from commit a16cb8709de7ddc6dd6ab71918af9fc5fd96f377)