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."
Merge pull request #125 from ClimbsRocks/patch-3

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