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.