sched: Fix various typos in comments

Go over the scheduler source code and fix common typos
in comments - and a typo in an actual variable name.

No change in functionality intended.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -2857,7 +2857,7 @@ unsigned long nr_running(void)
  * preemption, thus the result might have a time-of-check-to-time-of-use
  * race.  The caller is responsible to use it correctly, for example:
  *
- * - from a non-preemptable section (of course)
+ * - from a non-preemptible section (of course)
  *
  * - from a thread that is bound to a single CPU
  *