media: uapi book: Fix a few Sphinx warnings

Solve the following issues:

	Documentation/media/uapi/v4l/dev-sliced-vbi.rst:208: ERROR: Content block expected for the "raw" directive; none found.
	Documentation/media/uapi/v4l/dev-sliced-vbi.rst:216: ERROR: Content block expected for the "raw" directive; none found.
	Documentation/media/uapi/v4l/pixfmt-packed-rgb.rst:645: ERROR: Content block expected for the "raw" directive; none found.

Fixes: 70b074df

 ("media: fix pdf build with Spinx 1.6")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>