correct print output

Author: tomvanmele

src/compas/datastructures/tree/tree.py

@@ -244,6 +244,10 @@ class Tree(Datastructure):
     >>> branch.add(leaf2)
     >>> print(tree)
     <Tree with 4 nodes>
+        └── <TreeNode: root>
+            └── <TreeNode: branch>
+                ├── <TreeNode: leaf1>
+                └── <TreeNode: leaf2>
 
     """