update README.md

README.md

@@ -237,7 +237,7 @@ drop-in replacement for `property`.
 It is possible to create groups of `znflow.nodify` or `znflow.Nodes` independent
 from the graph structure. To create a group you can use
 `with graph.group(<name>)`. To access the group members, use
-`graph.get_group(<name>) -> list`.
+`graph.get_group(<name>) -> znflow.Group`.
 
 ```python
 import znflow