update readme of transformer benchmark (#4997)

PaddleNLP/benchmark/transformer/README.md

@@ -20,5 +20,5 @@ python3 train.py
 ``` shell
 cd dygraph/
 export CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
-python3 -m paddle.distributed.launch --selected_gpus=0,1,2,3,4,5,6,7 train.py
+python3 -m paddle.distributed.launch --gpus "0,1,2,3,4,5,6,7" train.py
 ```

PaddleNLP/examples/machine_translation/transformer/README.md

@@ -66,7 +66,7 @@ python train.py
 
 ```sh
 export CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
-python -m paddle.distributed.launch --selected_gpus=0,1,2,3,4,5,6,7 train.py
+python -m paddle.distributed.launch --gpus "0,1,2,3,4,5,6,7" train.py
 ```