[media] DocBook media: fix typo

The reference to v4l2-event-source-change should have been v4l2-event-src-change.
This caused a failure when building the spec.

Fixed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>