Fixes rSt typo;

Changelog

@@ -20,7 +20,7 @@ an overview of what's new in Celery 4.0.
 
 See :ref:`whatsnew-4.0` (in :file:`docs/whatsnew-4.0`).
 
-.. _version-4.0.0rc7
+.. _version-4.0.0rc7:
 
 4.0.0rc7
 ========