V4L/DVB (13680a): DocBook/media: copy images after building HTML

The rule for %.html removes the output directory, so there is no point
in copying images before building HTML.

Documentation/DocBook/Makefile |   10 +++++-----

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>