Update gym-unity/README.md

Co-authored-by: Chris Elion <chris.elion@unity3d.com>

Author: dongruoping

gym-unity/README.md

@@ -31,7 +31,7 @@ from the root of the project repository use:
 ```python
 from gym_unity.envs import UnityToGymWrapper
 
-env = UnityToGymWrapper(unity_environment, uint8_visual, flatten_branched, allow_multiple_obs)
+env = UnityToGymWrapper(unity_environment, uint8_visual, flatten_branches, allow_multiple_obs)
 ```
 
 - `unity_environment` refers to the Unity environment to be wrapped.