Fix typo "leaning_rate" (#324)

python/PPO.ipynb

     "hyperparameter_dict = {'max_steps':max_steps, 'run_path':run_path, 'env_name':env_name,\n",
     "    'curriculum_file':curriculum_file, 'gamma':gamma, 'lambd':lambd, 'time_horizon':time_horizon,\n",
     "    'beta':beta, 'num_epoch':num_epoch, 'epsilon':epsilon, 'buffe_size':buffer_size,\n",
-    "    'leaning_rate':learning_rate, 'hidden_units':hidden_units, 'batch_size':batch_size}"
+    "    'learning_rate':learning_rate, 'hidden_units':hidden_units, 'batch_size':batch_size}"
    ]
   },
   {