Updated links in bonus section

[jtspinks]

Wasn't sure if you want the cover3.jpg image coming from http://data.pr4e.org/cover3.jpg server location or https://www.py4e.com/cover3.jpg moving forward with the book?

I did notice when using the curl command on my Mac OS, the cover3.jpg file only shows up in my bash terminal if I use the web protocol of "https" for the www.py4e.com URL. Using the data.pr4e.org URL works fine as written and matches the URL used in the curl1.py and curl2.py code files in the book.