Chp 06 v4 update (#205)

* Updating Chp_06

Updating to PyMC v4

* Updating README

* Fixing isort pre-commit fail

Author: yahrMason

Rethinking_2/Chp_06.ipynb

@@ -45,7 +45,7 @@ from the root directory.
 
 ## PyMC v4 Updates
 
-Work is on-going to update these notebooks to be compatible with PyMC v4. Currently chapters 2 through 5 have been converted.
+Work is on-going to update these notebooks to be compatible with PyMC v4. Currently chapters 2 through 6 have been converted.
 
 If you wish to run these notebooks, please create the v4 environment by running: