Documentation: cgroup: Fix 'cgroup-legacy' -> 'cgroup-v1'

This should have happened in 6255c46f (cgroup: rename cgroup
documentations, 2016-01-11).

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index 65b3eac..9ae148a 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -7,7 +7,7 @@
 conventions of cgroup v2.  It describes all userland-visible aspects
 of cgroup including core and specific controller behaviors.  All
 future changes must be reflected in this document.  Documentation for
-v1 is available under Documentation/cgroup-legacy/.
+v1 is available under Documentation/cgroup-v1/.
 
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