Update v4 det model path in README.md

[tengxniu]

中英文超轻量PP-OCRv4检测模型的训练模型下载地址不对，根据model_list里面的地址进行了更新

[paddle-bot]

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[shiyutang]

你好，需要签署下CLA～

[andyjiang1116]

@tengxniu 感谢提出问题，辛苦把这个PR提到dygraph分支一份吧