doc: Fix typo "CONFIG_CGROUP_CGROUP_MEMCG_SWAP"

Signed-off-by: NPaul Bolle <pebolle@tiscali.nl>
Acked-by: NRob Landley <rob@landley.net>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

Documentation/cgroups/memory.txt

@@ -194,7 +194,7 @@ the cgroup that brought it in -- this will happen on memory pressure).
 But see section 8.2: when moving a task to another cgroup, its pages may
 be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.
 
-Exception: If CONFIG_CGROUP_CGROUP_MEMCG_SWAP is not used.
+Exception: If CONFIG_MEMCG_SWAP is not used.
 When you do swapoff and make swapped-out pages of shmem(tmpfs) to
 be backed into memory in force, charges for pages are accounted against the
 caller of swapoff rather than the users of shmem.