Comment # 2 on bug 99492 from Jani Nikula

Fix pushed to igt:

commit 95bfb2902473b9f4e644c3eb831fdf110d87ed4f
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Tue Jun 12 17:02:11 2018 +0300

    tools/intel_gpu_frequency: fix usage and examples