sched: fix kernel/acct.c comment

fix kernel/acct.c comment.

noticed by Lin Tan. Comment suggested by Olaf Kirch.

also see:

  http://bugzilla.kernel.org/show_bug.cgi?id=8220

Reported-by:  <tammy000@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>