doc: Fix misnamed FUTEX_CMP_REQUEUE_PI op constants

FUTEX_CMP_REQUEUE_PI was misnamed in two different ways:
FUTEX_REQUEUE_CMP_PI and FUTEX_REQUEUE_PI. The existence of two
different misnamings leaves the reader wondering if we are talking
about two different operations. Furthermore, the misnamings mean
that grepping the source for the correct name (which doesn't
appear at all) won't find this documentation file.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Reviewed-by: Darren Hart <darren@dvhart.com>
Link: http://lkml.kernel.org/r/54B9663D.9070000@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>