Update async_executor.md

doc/fluid/design/async_executor/async_executor.md

@@ -13,10 +13,7 @@ def train_loop():
     avg_cost, acc, prediction = bow_net(data, label)
     sgd_optimizer = fluid.optimizer.Adagrad(learning_rate=0.002)
     opt_ops, weight_and_grad = sgd_optimizer.minimize(avg_cost)
-    place = fluid.CPUPlace()
-    executor = fluid.Executor(place)
-    executor.run(startup_program)
-    async_executor = fluid.AsyncExecutor(place)
+    async_executor = fluid.AsyncExecutor()
     async_executor.run_startup_program(fluid.default_startup_program())
     epochs = 10
     for i in range(epochs):