rename instances of github.com to github.io in the quickstart guide

jekyll quickstart refers to USERNAME.github.com, should be USERNAME.github.io

Looking at the quickstart, the guide recommends USERNAME.github.com
http://jekyllbootstrap.com/usage/jekyll-quick-start.html

Since April, github is transitioning github page users to use github.io.
https://github.com/blog/1452-new-github-pages-domain-github-io