sched: Fix some typos in include/linux/preempt.h

There are some typos in the comments of migrate in
include/linux/preempt.h:

  elegible -> eligible
  it's -> its
  migirate_disable -> migrate_disable
  abritrary -> arbitrary

Just fix them.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>