Fixes instructions for disabling Apple NTP.

The best method for disabling Apple's NTP is via the Date & Time Preferences. If "Set date and time automatically" is unchecked, then Apple's NTP is disabled, and there's no need to do anything with launchctl. If it's checked, then any shutdown of Apple's NTP via launchctl will be undone the next time Date & Time Preferences is used. Thus, launchctl is never the best approach.