Backport PR matplotlib#19786: Fix code-of-conduct link on doc homepage.

Author: timhoffm

doc/index.rst

@@ -78,7 +78,8 @@ Join our community!
 ~~~~~~~~~~~~~~~~~~~
 
 Matplotlib is a welcoming, inclusive project, and everyone within the community
-is expected to abide by our `code of conduct <../CODE_OF_CONDUCT.md>`_.
+is expected to abide by our `code of conduct
+<https://github.com/matplotlib/matplotlib/blob/master/CODE_OF_CONDUCT.md>`_.
 
 
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