Fix link to PR in changelog

Author: lesteve

CHANGES.md

@@ -9,7 +9,7 @@
   Dropping support for older Python versions made it possible to simplify the
   code base signficantly, hopefully making it easier to contribute to and
   maintain the project.
-  ([PR #515](https://github.com/cloudpipe/cloudpickle/pull/515))
+  ([PR #517](https://github.com/cloudpipe/cloudpickle/pull/517))
 
 - Fix pickling of dataclasses and their instances.
   ([issue #386](https://github.com/cloudpipe/cloudpickle/issues/386),