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

This should have happened in 6255c46f (cgroup: rename cgroup
documentations, 2016-01-11).
Signed-off-by: NW. Trevor King <wking@tremily.us>
Signed-off-by: NTejun Heo <tj@kernel.org>

Documentation/cgroup-v2.txt

@@ -7,7 +7,7 @@ This is the authoritative documentation on the design, interface and
 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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