docs: thermal: time_in_state is displayed in msec and not usertime

The sysfs stats for cooling devices shows the time_in_state in msec,
remove the unwanted usertime comment.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/d5461bdf9ab6b6fee7f28f538582edbb426aa077.1605004905.git.viresh.kumar@linaro.org