media: doc: dev-encoder: Fixup whitespace before bold asterisks

An extra whitespace after the asterisks prevents bold syntax parsing
and results in the asterisks shown in the (non-bold) title.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>