Documentation/cgroups/memory.txt: fix stat file documentation

Documentation for inactive_anon / active_anon was mixed up. Fix that.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nsn.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>