Fix typos on quickstart page, fixes #1

I had original created #1 before I realized there was already a [commit](https://github.com/excaliburjs/excaliburjs.github.io/commit/ce7d171426706a6b6c6b1063fae39456623692dc) that existed for it. Some of the more nitpicky items remain in this PR.

Also note I didn't intend to remove trailing spaces in this PR, my text editor did it automatically upon save. I can remove those changes and rebase if necessary.