Fixed misspelling and added small example

Hi,

Loving your Ember on Rails tutorial. It's exactly what I've been looking for: a tutorial which puts the two together.

This is my first fork and pull request ever--first contribution ever--so if I'm doing things wrong I appreciate any feedback you provide. 

This pull request has "ned" fixed to "need" and a small example of something you mentioned could be done in the text, but which I had to figure out (good learning experience, I suppose).

Thanks again!