Fix a typo in Python-API.md (#4179)

Fix behavior_spec to behavior_specs

docs/Python-API.md

 env = UnityEnvironment(file_name="3DBall", seed=1, side_channels=[])
 # Start interacting with the evironment.
 env.reset()
-behavior_names = env.behavior_spec.keys()
+behavior_names = env.behavior_specs.keys()
 ...
 ```
 **NOTE:** Please read [Interacting with a Unity Environment](#interacting-with-a-unity-environment)