Fix link to base.css and bg.png

Fix for [issue 555](https://github.com/addyosmani/backbone-fundamentals/issues/555).

Similar solution than this [fix for chapter 4](https://github.com/addyosmani/backbone-fundamentals/commit/5385c01134c5fb52919db2da1ff483e2179e9195#diff-760873eb1fb422a57aefb9a9f3fa7635).