Update the example for plotting data points of various colors

[seisman]

Description of the problem

In the last plotting example in https://www.pygmt.org/latest/tutorials/plot.html, color=data.depth_km / data.depth_km.max() is passed to Figure.plot because makecpt wasn't implemented at that time.

We normalize the data values given to color because, by default, plot can only interpret values between 0 and 1. To use the actual data values, we would need to create a color palette table (CPT) which isn’t implemented yet.

Now we already can create a CPT file using pygmt.makecpt and we should rewrite the example using pygmt.makecpt.