[media] DocBook media: Fix a typo in encoder cmd

A copy-paste from DECODER_CMD : replace decoded by encoded.

Signed-off-by: Jean-Michel Hautbois <jean-michel.hautbois@veo-labs.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>