manual: Rework makedevs syntax

State that it should be a space separated list and fix the example.
Also reduce the number of dash used to 4.

[Thomas: replace tab by space, as noted by Yann.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>