Correct external link to cgroup documentation

The cgroup-v1 documentation has moved, correct the link.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/concepts/robustness.md

@@ -811,7 +811,7 @@ the filesystem is
 
 [kernel-proc]: http://www.kernel.org/doc/Documentation/filesystems/proc.txt
 
-[memory.usage_in_bytes]: http://www.kernel.org/doc/Documentation/cgroups/memory.txt
+[memory.usage_in_bytes]: http://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt
 
 [NETPRIO_CGROUP]: http://lwn.net/Articles/474695/