Change wording for spectrum example, use hot colormap for spectrogram

The `jet` colormap is for bipolar data (range of [−1, +1]), while spectrogram amplitude is unipolar (range of [0, +1]), so a sequential map like `hot` is a better choice.

http://www.jwave.vt.edu/~rkriz/Projects/create_color_table/color_07.pdf