doc: trace: fix reference to cpuidle documentation file

Old cpuidle/sysfs.txt file was replaced in aa5eee35. So, refer to
an updated file.

Fixes: aa5eee35

 (Documentation: admin-guide: PM: Add cpuidle document)
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>