[media] DocBook/dvbproperty.xml: Fix the units for DTV_FREQUENCY

The units for DTV_FREQUENCY are kHz for satellital delivery systems
(DVB-S/DVB-S2/DVB-TURBO/ISDB-S). Fix it at the API spec.

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