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>

include/linux/sched.h

@@ -516,7 +516,7 @@ struct thread_group_cputimer {
 struct autogroup;
 
 /*
- * NOTE! "signal_struct" does not have it's own
+ * NOTE! "signal_struct" does not have its own
  * locking, because a shared signal_struct always
  * implies a shared sighand_struct, so locking
  * sighand_struct is always a proper superset of