doc: cgroup: improve formatting of io example

We need a literal section, like few paragraphs below.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20200228000653.1572553-4-kuba@kernel.org


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