media: vivid.rst: add a blank line to correct ReST format

On all vivid parameters, there's an space after the parameter,
except for "DV Timings Signal Mode". That makes this single one
to be written in bold, and, at PDF output, at the same line as
its description.

Use the same convention as the other parameters, in order to
adjust its output.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>