Corrected documentation related to andReturnValues() method

I've been using mockery more and more over the last few months and the few times I decided I wanted to pass an array to andReturn() I kept getting fooled by the documentation, which currently states that one should use andReturns(), which doesn't actually exist.

So, I figured I'd use this as an opportunity to fork, clone, fix, add, commit, push, and fire off a pull request for the first time with a public repository.  Hope I don't screw things up...!  ;-)