ABI: sysfs-devices-system-cpu: remove a broken reference

This file doesn't exist anymore:
	Documentation/cpu-freq/user-guide.txt

As the ABI already points to Documentation/cpu-freq, just
remove the broken link and the associated text.
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: NJonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -238,9 +238,6 @@ Description:	Discover and change clock speed of CPUs
 
 		See files in Documentation/cpu-freq/ for more information.
 
-		In particular, read Documentation/cpu-freq/user-guide.txt
-		to learn how to control the knobs.
-
 
 What:		/sys/devices/system/cpu/cpu#/cpufreq/freqdomain_cpus
 Date:		June 2013