fixes broken binder link (#4929)

Author: Meenal Jhajharia

README.rst

@@ -11,7 +11,7 @@ algorithms. Its flexibility and extensibility make it applicable to a
 large suite of problems.
 
 Check out the `getting started guide <http://docs.pymc.io/notebooks/getting_started>`__,  or
-`interact with live examples <https://mybinder.org/v2/gh/pymc-devs/pymc3/master?filepath=%2Fdocs%2Fsource%2Fnotebooks>`__
+`interact with live examples <https://mybinder.org/v2/gh/pymc-devs/pymc3/main?filepath=%2Fdocs%2Fsource%2Fnotebooks>`__
 using Binder!
 For questions on PyMC3, head on over to our `PyMC Discourse <https://discourse.pymc.io/>`__ forum.