cgroup: Update documentation reference

The cgroup_subsys structure references a documentation file that has been
renamed after the v1/v2 split.  Since the v2 documentation doesn't
currently contain any information on kernel interfaces for controllers,
point the user to the v1 docs.

Cc: Tejun Heo <tj@kernel.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup-defs.h

@@ -561,7 +561,7 @@ struct cftype {
 
 /*
  * Control Group subsystem type.
- * See Documentation/cgroups/cgroups.txt for details
+ * See Documentation/cgroup-v1/cgroups.txt for details
  */
 struct cgroup_subsys {
 	struct cgroup_subsys_state *(*css_alloc)(struct cgroup_subsys_state *parent_css);