[media] DocBook: Use constant tag for monospaced fonts

[media] DocBook: Use constant tag for monospaced fonts

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