Documentation: admin-guide: intel_pstate: Fix sysfs path

Fix an incorrect sysfs path in the intel_pstate admin-guide
documentation.

Fixes: 33fc30b4 (cpufreq: intel_pstate: Document the current behavior and user interface)
Reported-by: Pawit Pornkitprasan <p.pawit@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>