Fix title underline in README

This prevented us from releasing to pypi

Author: Lorak-mmk

README.rst

@@ -70,7 +70,7 @@ Contributing
 See `CONTRIBUTING <https://github.com/scylladb/python-driver/blob/master/CONTRIBUTING.rst>`_.
 
 Error Handling
-------------
+--------------
 While originally written for the Java driver, users may reference the `Cassandra error handling done right blog <https://www.datastax.com/blog/cassandra-error-handling-done-right>`_ for resolving error handling scenarios with Apache Cassandra.
 
 Reporting Problems