Add a missing Sphinx reference to a class

Author: EliahKagan

Diff for: git/repo/base.py

@@ -1452,7 +1452,7 @@ def clone(
 
         :param kwargs:
             * ``odbt`` = ObjectDatabase Type, allowing to determine the object database
-              implementation used by the returned Repo instance.
+              implementation used by the returned :class:`Repo` instance.
             * All remaining keyword arguments are given to the ``git clone`` command.
 
         :return: