Correct typos and grammar, general improvements

This is my usual typo-correcting pull request. I've also added some additional polish to the spec.

One part worth noting: the spec reads "When you zoom in on a state, the circle radius should also 'zoom in' and stay the same size relative to the state's size." I've clarified that this is [geometric zoom](http://bl.ocks.org/mbostock/3680999). If you meant for this to instead be [sematic zoom](http://bl.ocks.org/mbostock/3680957), please let me know and I can make that change.