IRQ.txt: add a markup for its title

This simple document only needs a markup for its title to be
using the standard we're adopting for text documents.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/IRQ.txt

+===============
 What is an IRQ?
+===============
 
 An IRQ is an interrupt request from a device.
 Currently they can come in over a pin, or over a packet.