media: media-types.rst: fix doc warnings

Fix these warnings when building the documentation:

media.h.rst:6: WARNING: undefined label: media-ent-f-dv-decoder (if the link has no caption the label must precede a section header)
media.h.rst:6: WARNING: undefined label: media-ent-f-dv-encoder (if the link has no caption the label must precede a section header)
media.h.rst:6: WARNING: undefined label: media-ent-f-dv-decoder (if the link has no caption the label must precede a section header)

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>