Documentation/x86: Rename IRQSTACKSIZE to IRQ_STACK_SIZE

The IRQSTACKSIZE was renamed to the IRQ_STACK_SIZE in the
(26f80bd6 x86-64: Convert irqstacks to per-cpu) commit,
but it still named IRQSTACKSIZE in the documentation. This
patch fixes this.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>