Update 2012-11-08-how-to-squash-commits-in-a-github-pull-request.md

"git rebase -i" doesn't do anything. Oops.
Merge pull request #21 from jlsuttles/how-to-squash-fix

Update 2012-11-08-how-to-squash-commits-in-a-github-pull-request.md