Fixes broken link in BIDS example (see bids-standard/bids-bep016#22)

examples/plot_bids_layout.py

@@ -14,7 +14,7 @@
     described in the existing BIDS specification, but describing these
     is part of an ongoing effort. Wherever possible, we conform with
     the draft implementation of the BIDS DWI derivatives available
-    `here <https://bids-specification.readthedocs.io/en/derivatives/05-derivatives/06-diffusion-derivatives.html>`_
+    `here <https://bids-specification.readthedocs.io/en/wip-derivatives/05-derivatives/05-diffusion-derivatives.html>`_
 
 In this example, we will explore the use of BIDS in pyAFQ and see
 how BIDS allows us to extend and provide flexibility to the users