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>

Documentation/cgroup-v1/memory.txt

@@ -524,9 +524,9 @@ Note:
 	Only anonymous and swap cache memory is listed as part of 'rss' stat.
 	This should not be confused with the true 'resident set size' or the
 	amount of physical memory used by the cgroup.
-	'rss + file_mapped" will give you resident set size of cgroup.
+	'rss + mapped_file" will give you resident set size of cgroup.
 	(Note: file and shmem may be shared among other cgroups. In that case,
-	 file_mapped is accounted only when the memory cgroup is owner of page
+	 mapped_file is accounted only when the memory cgroup is owner of page
 	 cache.)
 
 5.3 swappiness