Wording and grammar changes

There were a few places where the existing phrase caused me to stumble a bit, which I have reworded slightly. Also some grammar or spelling fixes.

There are two example HTML files that have script tags for Underscore and Backbone but not jQuery - and then use $. I have added jQuery to those examples.

Doing this in stages, so these are all of my 'nits' up to the end of chapter 3.