Fix changelog headings

Author: sloria

CHANGELOG.rst

@@ -2,30 +2,30 @@ Changelog
 ---------
 
 2.20.3 (2019-09-04)
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++++++++++++++++++++
 
 Bug fixes:
 
 - Don't swallow ``TypeError`` exceptions raised by ``Field._bind_to_schema`` or ``Schema.on_bind_field``.
 
 2.20.2 (2019-08-20)
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++++++++++++++++++++
 
 Bug fixes:
 
 - Prevent warning about importing from ``collections`` on Python 3.7
   (:pr:`1354`). Thanks :user:`nicktimko` for the PR.
 
 2.20.1 (2019-08-13)
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++++++++++++++++++++
 
 Bug fixes:
 
 - Fix bug that raised ``TypeError`` when invalid data type is
   passed to a nested schema with ``@validates`` (:issue:`1342`).
 
 2.20.0 (2019-08-10)
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++++++++++++++++++++
 
 Bug fixes: