Documentation: admin-guide: PM: Fix two typos

Fix two typos in the admin-guide:

 - a missing e in "reference_perf" in cppc_sysfs.rst.
 - the amd_pstate sysfs path uses a dash instead of an underscore.

Signed-off-by: Erwan Velu <e.velu@criteo.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>