[docs] Fix a configuration error in RollerBall that doesn't allow training to run. (#3945)

mlagents.trainers.exception.UnityTrainerException: The hyper-parameter memory_size could not be found for the <class 'mlagents.trainers.ppo.trainer.PPOTrainer'> trainer of brain RollerBall.

docs/Learning-Environment-Create-New.md

     learning_rate: 3.0e-4
     learning_rate_schedule: linear
     max_steps: 5.0e4
+    memory_size: 128
     normalize: false
     num_epoch: 3
     num_layers: 2