Updated example doc for SubprocVecEnv (#531)

* Wrapped SubprocVecEnv example with main check

* Update examples.rst

docs/guide/examples.rst

@@ -117,19 +117,20 @@ Multiprocessing: Unleashing the Power of Vectorized Environments
       set_global_seeds(seed)
       return _init
 
-  env_id = "CartPole-v1"
-  num_cpu = 4  # Number of processes to use
-  # Create the vectorized environment
-  env = SubprocVecEnv([make_env(env_id, i) for i in range(num_cpu)])
-
-  model = ACKTR(MlpPolicy, env, verbose=1)
-  model.learn(total_timesteps=25000)
-
-  obs = env.reset()
-  for _ in range(1000):
-      action, _states = model.predict(obs)
-      obs, rewards, dones, info = env.step(action)
-      env.render()
+  if __name__ == '__main__':
+      env_id = "CartPole-v1"
+      num_cpu = 4  # Number of processes to use
+      # Create the vectorized environment
+      env = SubprocVecEnv([make_env(env_id, i) for i in range(num_cpu)])
+
+      model = ACKTR(MlpPolicy, env, verbose=1)
+      model.learn(total_timesteps=25000)
+
+      obs = env.reset()
+      for _ in range(1000):
+          action, _states = model.predict(obs)
+          obs, rewards, dones, info = env.step(action)
+          env.render()