nommu-mmap.txt: don't use all upper case on titles

This file is almost in the standard format we're adopting for
other documentation text files. Yet, it use upper case on
titles.

So, in order to uniform how chapter names, adjust caps on
titles.

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