fix import

examples/simple_example_3.py

@@ -3,7 +3,7 @@ import random
 from flatland.envs.generators import random_rail_generator
 from flatland.envs.rail_env import RailEnv
 from flatland.utils.rendertools import RenderTool
-from flatland.core.env_observation_builder import TreeObsForRailEnv
+from flatland.envs.observations import TreeObsForRailEnv
 import numpy as np
 
 random.seed(100)