Verify URLs are working

[msoltadeo]

@abrilr1604, we had made the repo public and deployed it on a public website.
Could you verify that every link is working correctly. You might not remember, but sometime ago, in preparation for this moment, we changed all the urls that made reference to sections in the course for cross references. I am mentioning this because thanks to that, most of the links should be working.

If you find issues, please create a branch and make a PR

[abrilr1604]

@msoltadeo I just reviewed all the links in the course again. All cross references are working correctly! the issues I found with some links are the same as those mentioned in my last comment on issue #111