Add discussions thread link to Discussions-To

Author: msarahan

peps/pep-0766.rst

@@ -3,7 +3,7 @@ Title: Standardizing selection of Python package files across multiple sources
 Author: Michael Sarahan, msarahan@gmail.com
 Sponsor: Barry Warsaw, barry@python.org
 PEP-Delegate: <PEP delegate’s real name>
-Discussions-To: <REQUIRED: URL of current canonical discussion thread>
+Discussions-To: https://discuss.python.org/t/pep-for-handling-multiple-indexes-index-priority/71589
 Status: Draft
 Type: Standards Track
 Topic: Packaging