edit README.rst

Slight change to simplify text.

Author: DerekRoy

README.rst

@@ -6,7 +6,7 @@
 |Build Status| |Coverage| |NumFOCUS_badge| |Binder|
 
 PyMC3 is a Python package for Bayesian statistical modeling and Probabilistic Machine Learning
-which focuses on advanced Markov chain Monte Carlo and variational fitting
+focusing on advanced Markov chain, Monte Carlo, and variational fitting
 algorithms. Its flexibility and extensibility make it applicable to a
 large suite of problems.