Fixing dead links

This pull request removes a reference to a dead link `http://html5up.net/uploads/previews/6742121165068310.jpg`, as well as fixes a reference display of HTML To Jade converter.

Also, there is one more dead link under the `How do I use Socket.io with Hackathon Starter` section, right before `Mongoose Cheatsheet` section. 

There is a reference to `http://hackathonstarter.herokuapp.com/dashboard`, which leads to `Cannot GET /dashboard ` error. I left this link as it was, as I am not sure where you would like to redirect this link. I will leave that up to you, just wanted to bring that up as well.