Fix type hint on create_tag

pycharm yells at me without this

git/repo/base.py

@@ -482,7 +482,7 @@ def delete_head(self, *heads: "Union[str, Head]", **kwargs: Any) -> None:
     def create_tag(
         self,
         path: PathLike,
-        ref: str = "HEAD",
+        ref: Union[str, 'SymbolicReference'] = "HEAD",
         message: Optional[str] = None,
         force: bool = False,
         **kwargs: Any,