fix typo

Hi, Mike!

Sorry for rewriting history in previous pull request: I google every time when I want to update a my repo and the search results are changed)))

 http://stackoverflow.com/questions/7244321/how-to-update-github-forked-repository.

So, the new guide do "git rebase upstream/master" insead of "git merge upstream/master".

PS It's time to write a shell script for updating my repo...