Documentation: fix admin-guide doc refs

Make admin-guide document refs valid.

Signed-off-by: Tom Saeger <tom.saeger@oracle.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ABI/stable/sysfs-devices

@@ -1,5 +1,5 @@
 # Note: This documents additional properties of any device beyond what
-# is documented in Documentation/sysfs-rules.txt
+# is documented in Documentation/admin-guide/sysfs-rules.rst
 
 What:		/sys/devices/*/of_node
 Date:		February 2015