add display tree

app.py

@@ -137,12 +137,12 @@ def insert(self, val):
 """
 
 display_code = r"""
-def printValues(self):
+def display_tree(self):
     if self.left:
-        self.left.printValues()
+        self.left.display_tree()
     print(self.val)
     if self.right:
-        self.right.printValues()
+        self.right.display_tree()
 """
                         
 def docgen_func(function_code):