Fixed typo in folder URL for the 2nd edition of Statistical Rethinking (#71)

Author: deanwampler

README.md

@@ -2,7 +2,7 @@
 PyMC3 educational resources, including PyMC3 port of books (original models in STAN/BUGS/JAGS etc,.), PyMC3 talk slides, case studies, and other resources.
 
 -  [PyMC3 port of the book "Statistical Rethinking" by Richard McElreath (first edition)](https://github.com/pymc-devs/resources/tree/master/Rethinking)
--  [PyMC3 port of the book "Statistical Rethinking" by Richard McElreath (second edition)](https://github.com/pymc-devs/resources/tree/master/Rethinking2)
+-  [PyMC3 port of the book "Statistical Rethinking" by Richard McElreath (second edition)](https://github.com/pymc-devs/resources/tree/master/Rethinking_2)
 -  [PyMC3 port of the book "Bayesian Cognitive Modeling" by Michael Lee and EJ Wagenmakers](https://github.com/pymc-devs/resources/tree/master/BCM)
 -  [PyMC3 port of the book "Bayesian Statistical Methods" by Brian J. Reich and Sujit K. Ghosh](https://github.com/pymc-devs/resources/tree/master/BSM)
 -  [PyMC3 port of the book "Bayesian Data Analysis" by Andrew Gelman, John B. Carlin, Hal S. Stern, David B. Dunson, Aki Vehtari and Donald B. Rubin](https://github.com/pymc-devs/resources/tree/master/BDA3)