Created by: shanyi15

检查发现以下几处有死链，请修复 1. https://github.com/PaddlePaddle/book/blob/develop/03.image_classification/README.cn.md The Pascal Visual Object Classes Challenge: A Retrospective 链接错误

https://github.com/PaddlePaddle/book/blob/develop/05.recommender_system/README.cn.md Hybrid Web Recommender Systems

https://github.com/PaddlePaddle/book/blob/develop/06.understand_sentiment/README.cn.md sequence_conv_pool

https://github.com/PaddlePaddle/book/blob/develop/07.label_semantic_roles/README.cn.md Conditional random fields: Probabilistic models for segmenting and labeling sequence data

https://github.com/PaddlePaddle/book/blob/develop/08.machine_translation/README.cn.md WMT-14 bitexts(after selection) dev+test data 较小规模的数据集