libs/utils/trace: Cleanup docstring
libs/utils/trace: Cleanup docstring

- This refers to a specific CPU (not "at least one")
- This refers to cpuidle not cpufreq

ARM-software/lisa,joelagnel/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,mdigiorgio/lisa,joelagnel/lisa,credp/lisa,arnoldlu/lisa,arnoldlu/lisa,bjackman/lisa,mdigiorgio/lisa,credp/lisa,bjackman/lisa,ARM-software/lisa