media: dvb-core/demux.h: fix kernel-doc warning

Fix this kernel-doc warning:

WARNING: kernel-doc 'media-git/scripts/kernel-doc -rst -enable-lineno media-git/drivers/media/dvb-core/demux.h' processing failed with: 'ascii' codec can't decode byte 0xe2 in position 6368: ordinal
not in range(128)

Caused by using fancy quotes instead of regular quotes.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>