[heavy_tails] fix exercise issue

[jstac]

Ex 3 in https://intro.quantecon.org/heavy_tails.html is broken.

It should be asking the reader to replicate one of the figures, in the section "Simulations", not Ex 1.

This will be easier if the figure has a caption and a label.

@chappiewuzefan @DiaPorntipa could one of you please fix it?

[chappiewuzefan]

Sure ~

[mmcky]

@HumphreyYang I know there was discussion in #270 around fixing this exercise. Would you mind to review this thread and create a PR?