genirq: Fix typo in documentation of enumeration field name

It should say IRQ_NESTED_THREAD instead of IRQ_NESTED_TRHEAD.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Link: http://lkml.kernel.org/r/1443704093-36837-1-git-send-email-mika.westerberg@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>