fix a comment fault, sorry

The formular for [velocity:ToHue: count:] method is based on process below:
d(v) = Vmax - v (v > Vmin)
Len(v) = Vmax-Vmin
Len(h) = Hmax - Hmin
Then this form should be true: d(v)/Len(v) = d(h)/Len(h)
finally: H(v) = Hmin + d(h)