Fix gradient descent example for Python 3

[naoyak]

The example producing Figure 8-3 in the book doesn't work in Python 3, apparently because the iterator returned by map() doesn't work with pyplot, so using a list comprehension here instead.

[daun-io]

I was finding the way to fix this by including map into list which is absolutely working but not an efficient idea.
Thank you for suggesting me a better way :)

[brettmendes87]

Just wanted to say thank you to naoyak! This one really had me scratching my head when I first encountered it.