Link to createspace purchase page broken

[andrewcrisp]

At URL https://natureofcode.com/book/, button labelled "Buy from CreateSpace" linking to https://www.createspace.com/4068419 gets redirected to the CreateSpace homepage https://www.createspace.com/

[shiffman]

Thanks for the report! CreateSpace was bought by Amazon so there is no longer a separate direct purchase. I'll address this in the new website! Preview here: https://nature-of-code-2nd-edition.netlify.app/