Link to derivatives.json wrong in tutorial

[gladomat]

There's a dead link to the derivatives.json file in the tutorial.

[KirstieJane]

Here's the new link! It happened because grabbids was renamed to layout!

https://github.com/INCF/pybids/blob/master/bids/layout/config/derivatives.json

[tyarkoni]

Closing this because derivatives are now handled differently at initialization. I've opened a more general issue (#259) for this, as we need to sweep through all the docs and make sure they play nice with the latest changes.