Fixed header underlines in performance docs.
@@ -411,7 +411,7 @@ performance gains for your application to outweigh the potential risks.
With these caveats in mind, you should be aware of:
`PyPy <https://www.pypy.org/>`_
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`PyPy <https://www.pypy.org/>`_ is an implementation of Python in Python itself
(the 'standard' Python implementation is in C). PyPy can offer substantial
