text: Title: Hyperlink has invalidated in DATASETS.md
  Body: I find the hyperlink in  DATASETS.md: https://github.com/KaiyangZhou/deep-person-reid/blame/master/DATASETS.md#L167-L168 has invalidated.
  I think you may modify it : 
  ```
  - [ImageDataset](https://github.com/KaiyangZhou/deep-person-reid/blob/master/torchreid/dataset_loader.py#L31)
  - [VideoDataset](https://github.com/KaiyangZhou/deep-person-reid/blob/master/torchreid/dataset_loader.py#L50)
  ```