Update file extensions on "tutorials" page

On the ["tutorials" page](http://usejsdoc.org/about-tutorials.html), there's a list of file types you can use to define a tutorial's configuration. The list should be updated to say that only `.json` is supported.

This issue came up on [Stack Overflow](http://stackoverflow.com/questions/22495502/how-do-i-set-a-jsdoc-tutorial-title).
