trace/events/sched.h: fix duplicated word

Change "It it" to "It is".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/25305c1d-4ee8-e091-d20f-e700ddad49fd@infradead.org