Improve bindings guide to use relative bindings

The use of global key paths is much less relevant today than it used to be, and this demonstration is therefore a bit outdated.

I think there may be space to talk about key paths being global when they start with a capital, but IMO this is not the place and just muddies the water.

This was triggered by [this SO question](https://stackoverflow.com/questions/20978904/why-does-a-simple-ember-js-binding-not-work/20978989).