As reminded by Jonathan, several places where emphasys
role="tt" were used are actually trying to change the font to
monospaced.

We do that, on other places, by using the constant tag.

So, use it here too.
Reported-by: NJonathan Corbet <corbet@lwn.net>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>