Add checkpoint file download link in doc (#8391)

* tiny add

* fix

Author: xiaoyuan0203

docs/en/2_new_data_model.md

@@ -243,6 +243,8 @@ data = dict(
 load_from = 'checkpoints/mask_rcnn_r50_caffe_fpn_mstrain-poly_3x_coco_bbox_mAP-0.408__segm_mAP-0.37_20200504_163245-42aa3d00.pth'
 ```
 
+This checkpoint file can be downloaded [here](https://download.openmmlab.com/mmdetection/v2.0/mask_rcnn/mask_rcnn_r50_caffe_fpn_mstrain-poly_3x_coco/mask_rcnn_r50_caffe_fpn_mstrain-poly_3x_coco_bbox_mAP-0.408__segm_mAP-0.37_20200504_163245-42aa3d00.pth)
+
 ## Train a new model
 
 To train a model with the new config, you can simply run