docs: use code instead of tt-tag

docs: use code instead of tt-tag

The tt-tag has been removed from HTML5, so let's normalize this to
code-tags intead. This just makes things a bit more consistent, as we've
mixed these left and right so far anyway.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>