typo in header

Author: CamDavidsonPilon

README.md

@@ -40,7 +40,7 @@ The below chapters are rendered via the *nbviewer* at
 [nbviewer.ipython.org/](http://nbviewer.ipython.org/), and is read-only and rendered in real-time.
 Interactive notebooks + examples can be downloaded by cloning! 
 
-#### PyMC3
+### PyMC2
 
 * [**Prologue:**](http://nbviewer.ipython.org/urls/raw.github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers/master/Prologue/Prologue.ipynb) Why we do it.