sched.h: Fix a typo ("its")

The sentence uses the possessive pronoun, which is spelled
without an apostrophe.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Cc: Jiri Kosina <trivial@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1300735487-2406-1-git-send-email-j.neuschaefer@gmx.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>