Removed dead link in release notes.

docs/releases/1.8.txt

@@ -742,5 +742,5 @@ will be removed in Django 2.0. Historically, it was used to handle fields where
 type conversion was needed when loading from the database, but it was not used
 in ``.values()`` calls or in aggregates. It has been replaced with
 :meth:`~django.db.models.Field.from_db_value`. Note that the new approach does
-not call the :meth:`~django.db.models.Field.to_python` method on assignment
-as was the case with ``SubfieldBase``.
+not call the ``to_python()`` method on assignment as was the case with
+``SubfieldBase``.