fix doc (#4857)

doc/doc_ch/distributed_training.md

@@ -13,7 +13,7 @@
 ```shell
 python3 -m paddle.distributed.launch \
     --log_dir=./log/ \
-    --gpus '0,1,2,3,4,5,6,7' \
+    --gpus "0,1,2,3,4,5,6,7" \
     tools/train.py \
     -c configs/rec/rec_mv3_none_bilstm_ctc.yml
 ```

doc/doc_en/distributed_training.md

@@ -13,7 +13,7 @@ Take recognition as an example. After the data is prepared locally, start the tr
 ```shell
 python3 -m paddle.distributed.launch \
     --log_dir=./log/ \
-    --gpus '0,1,2,3,4,5,6,7' \
+    --gpus "0,1,2,3,4,5,6,7" \
     tools/train.py \
     -c configs/rec/rec_mv3_none_bilstm_ctc.yml
 ```