Typo: Added 'is' to "if it not defined"

Added 'is' to the following sentence: "Every Backbone.js view has an "el" property, and if it is not defined, Backbone.js will construct its own, which is an empty div element."

![image](https://cloud.githubusercontent.com/assets/7017045/5586318/a858945c-907e-11e4-8e43-e1ec731daac0.png)