sched.c: fix kernel-doc for runqueue_is_locked()

Fix kernel-doc warning for runqueue_is_locked():

  Warning(kernel/sched.c:664): missing initial short description on line:

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>