fixed links to base.css & bg.png

asset folder has meanwhile been renamed to [site-assets](https://github.com/tastejs/todomvc/tree/gh-pages/site-assets/), and base.css is now [main.css](https://github.com/tastejs/todomvc/blob/gh-pages/site-assets/main.css).

Instead of linking to the new files, I changed the links to v1.1.0 which still has the old files that this book is working with.