PM / hibernate: Correct documentation

Correct the meaning of PM_HIBERNATION_PREPARE in the docs.

References: http://lkml.kernel.org/r/20130512162717.GA6305@pd.tnic


Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>