doc: cgroup: improve formatting

Fix tabs vs spaces issue which cases the line to be considered
a new list entry.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20200228000653.1572553-2-kuba@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>