fixed calculation method of "cv::TickMeter".

I think that the calculation method of the following function is incorrect.
* cv::TickMeter::getTimeMicro()
* cv::TickMeter::getTimeMilli()
* cv::TickMeter::getTimeSec()

> (double)getTimeTicks()/getTickFrequency();

Result of this calculation is "TimeSec". 
But, current implementation is regarded as the "TimeMicroSec".