docs: cpufreq: fix a broken reference

The old cpufreq docs got moved. Let's also update its
pointer inside Kconfig.x86.

Fixes: 03b2249 ("Documentation: cpufreq: Move legacy driver documentation")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/Kconfig.x86

@@ -25,7 +25,7 @@ config X86_PCC_CPUFREQ
 	  This driver adds support for the PCC interface.
 
 	  For details, take a look at:
-	  <file:Documentation/cpu-freq/pcc-cpufreq.txt>.
+	  <file:Documentation/admin-guide/pm/cpufreq_drivers.rst>.
 
 	  To compile this driver as a module, choose M here: the
 	  module will be called pcc-cpufreq.