按钮不更新在Ajax - Rails教程3§12.2.5

I'm going through the Rails Tutorial by Michael Hartl and hit a small snag at §12.2.5 where we're supposed to create a working button with Ajax. I know the code is correct (I wound up copying it directly from the book and retyping it three times) and I'm green. But it doesn't actually work!I'm going through the Rails Tutorial by Michael