Update the link to the Aprende a Programar con Ruby.

The link is outdated because the GitHub organization maintaining the book
changed their name. The link currently works because of a redirect, but it
would better to replace it with the current link.
Merge pull request #1010 from jbranchaud/update-aprende-a-programar-con-ruby

Update the link to the Aprende a Programar con Ruby.