Fix link-breaking typo (pypa#328)

Author: jwodder

source/index.rst

@@ -22,7 +22,7 @@ tools.
 This guide is maintained on `GitHub`_ by the `Python Packaging Authority`_. We
 happily accept any :doc:`contributions and feedback <contribute>`. 😊
 
-.. _GitHub: https://github.com/pypa/python-packaging-user-guide>
+.. _GitHub: https://github.com/pypa/python-packaging-user-guide
 .. _Python Packaging Authority: https://pypa.io
 
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