Fix wrong class name in git.objects.tag docstring

In 6126997 (#1850), I had meant to have the git.objects.tag module
docstring say that the module defined the TagObject class, to help
ensure no one would confuse this with the TagReference class. But I
instead had it wrongly say it defined the TagReference class! This
fixes that.

Author: EliahKagan

git/objects/tag.py

@@ -5,7 +5,7 @@
 
 """Provides an :class:`~git.objects.base.Object`-based type for annotated tags.
 
-This defines the :class:`TagReference` class, which represents annotated tags.
+This defines the :class:`TagObject` class, which represents annotated tags.
 For lightweight tags, see the :mod:`git.refs.tag` module.
 """