docs: staging: don't use literalinclude

Such directive cause troubles with PDF output.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9a9043face7ace90e3616ba840a8e09cfc8f143f.1592203650.git.mchehab+huawei@kernel.org


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