add PR link to whats-new.rst entry

Author: kmuehlbauer

doc/whats-new.rst

@@ -73,7 +73,7 @@ Internal Changes
 - Remove internal support for reading GRIB files through the ``cfgrib`` backend. ``cfgrib`` now uses the external
   backend interface, so no existing code should break.
   By `Deepak Cherian <https://github.com/dcherian>`_.
-- Implement CF coding functions in ``VariableCoders``.
+- Implement CF coding functions in ``VariableCoders`` (:pull:`7719`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_
 
 - Added a config.yml file with messages for the welcome bot when a Github user creates their first ever issue or pull request or has their first PR merged. (:issue:`7685`, :pull:`7685`)