media: Documentation: vivid.rst: fix confusing section refs

The documentation contained several instances of "section X"
references, which no longer map to whatever X was.

Replace these by the section titles.

Also fix a single confusing typo in the "Radio & RDS Looping" section:
"are regular frequency intervals" -> "at regular frequency intervals"

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>