Fix link for config file in FAQ

[JohannesWiesner]

The link to the config file is not valid anymore:

Only modifications of these queries will have any effect. You may filter on any entity defined in the PyBIDS config file. To select images that do not have the entity set, use json value: null. To select images that have any non-empty value for an entity use string: '*'

The new link is:

https://github.com/bids-standard/pybids/blob/master/src/bids/layout/config/bids.json