Update 2014-12-14-getting-started.md

* Fixed various typos
* changed five.Sensor("A0") to new five.Sensor("A0"), not because the other way didn't work, but because it's consistent with the other code doing new five.Board() and new five.Led(11).  I think it's best to keep consistent instead of explaining to someone why they are different and why they both happen to work.