docs/admin-guide/cgroup-v2: fix mount opt rendering

Due to an extra empty line between the option and its description
it is rendered not like in other places.

Remove the empty lines to fix.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20210120001824.385168-11-kolyshkin@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>