update distributed script in user guide ~

Author: HydrogenSulfate

docs/zh/user_guide.md

@@ -952,8 +952,8 @@ best_value: 0.02460772916674614
 
     ``` sh
     # 指定 0,1,2,3 张卡启动分布式数据并行训练
-    export CUDA_VISIBLE_DEVICES=0,1,2,3
-    python -m paddle.distributed.launch --gpus="0,1,2,3" poiseuille_flow.py
+    CUDA_VISIBLE_DEVICES=0,1,2,3 \
+        python -m paddle.distributed.launch poiseuille_flow.py
     ```
 
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