docs/admin-guide: cgroup-v2: typos and spaces

- fix a typo (mempry -> memory) in a file name;
- add space before "(" where appropriate.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20210120001824.385168-7-kolyshkin@gmail.com


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