tzsetup: Correct UTC description

[emaste]

Summary

UTC is Coordinated Universal Time, not Greenwich Mean Time.

[imp]

This is 100% correct, wearing my time geek hat.

GMT vs UTC is a confusing and fascinating topic, but the tl;dr is UTC is the preferred term and it stands for Coordinated Universal Time (the initials arbitrary to match neither the French nor the English initils).
GMT is the old name and pedantically differs from UTC in ways that aren't relevant for this. It's also ambiguous because it was used interchangeably with UTC after the slightly tweaked UTC system replaced the GMT system. It's best avoided, although NASA still uses GMT to keep it's British partners happy).

Ah, the politics of time names.