cpupower: fix reference to nonexistent document

This file was renamed from .txt to .rst and left a dangling reference.
Fix it.

Fixes: 151f4e2b

 ("docs: power: convert docs to ReST and rename to *.rst")
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Len Brown <len.brown@intel.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>