docs: staging: use small font for literal includes

The normal font is too big to display 80 columns, causing extra
breaks to be added at weird places.

change to the footnotesize, as this would fit a little bit
better.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/fe2cc675ec69bfa50aa7e183dcd911077046b01f.1592918949.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>