docs: cgroup-v1: add it to the admin-guide book

Those files belong to the admin guide, so add them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index 5623b99..4f18456 100644
--- a/Documentation/kernel-per-CPU-kthreads.txt
+++ b/Documentation/kernel-per-CPU-kthreads.txt
@@ -12,7 +12,7 @@
 
 -	Documentation/IRQ-affinity.txt:  Binding interrupts to sets of CPUs.
 
--	Documentation/cgroup-v1:  Using cgroups to bind tasks to sets of CPUs.
+-	Documentation/admin-guide/cgroup-v1:  Using cgroups to bind tasks to sets of CPUs.
 
 -	man taskset:  Using the taskset command to bind tasks to sets
 	of CPUs.