isapnp.txt: promote title level

This simple document only needs to promote its title to be
using the standard we're using for document texts.

Yet, IMHO, it would be worth merging this file with
Documentation/pnp.txt in the future.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>