update yolox readme

Author: Wish

README.md

@@ -194,7 +194,7 @@ model.head.decode_in_inference = True
 # wget https://github.com/Megvii-BaseDetection/YOLOX/releases/download/0.1.1rc0/yolox_m.pth
 
 # 导出模型
-python tools/export_onnx.py -c yolox_m.pth -f exps/default/yolox_m.py --output-name=yolox_m.onnx --dynamic
+python tools/export_onnx.py -c yolox_m.pth -f exps/default/yolox_m.py --output-name=yolox_m.onnx --dynamic --no-onnxsim
 ```
 
 4. 执行程序