docs: uio-howto.rst: use a code block to solve a warning

/devel/v4l/docs/Documentation/driver-api/uio-howto.rst:715: WARNING: Unexpected indentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>