Documentation: Fix 'file_mapped' -> 'mapped_file'

There is no entry file_mapped in the memory.stat file. This looks like a
simple word flip that's gone unnoticed since 2010 (dc10e281,
memcg: update documentation).

Signed-off-by: Florian Schmidt <florian.schmidt@neclab.eu>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>