doc: cgroup: improve formatting of cpuset examples

We need literal sections otherwise the entire example is rendered
as a single line.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20200228000653.1572553-5-kuba@kernel.org


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