Fix DPO link

Co-authored-by: Dustin Ingram <di@users.noreply.github.com>

Author: astrofrog

peps/pep-0771.rst

@@ -20,7 +20,7 @@ Motivation
 ==========
 
 Various use cases for default extras and possible solutions in this PEP were discussed
-extensively on `this DPO thread <https://discuss.python.org/t/adding-a-default-extra-require-environment/4898/38>`__.
+extensively on `this DPO thread <https://discuss.python.org/t/adding-a-default-extra-require-environment/4898>`__.
 These fall into two broad cases that that provide the
 motivation for the present PEP.