fix retinanet r50 2x model error

MODEL_ZOO.md

@@ -123,7 +123,7 @@ More models with different backbones will be added to the model zoo.
 | :-------------: | :-----: | :-----: | :------: | :-----------------: | :------------: | :----: | :------------------------------------------------------------------------------------------------------------------------------: |
 |    R-50-FPN     |  caffe  |   1x    |   3.4    |        0.285        |      12.5      |  35.8  |                                                                -                                                                 |
 |    R-50-FPN     | pytorch |   1x    |   3.6    |        0.308        |      12.1      |  35.6  |    [model](https://s3.ap-northeast-2.amazonaws.com/open-mmlab/mmdetection/models/retinanet_r50_fpn_1x_20181125-7b0c2548.pth)     |
-|    R-50-FPN     | pytorch |   2x    |    -     |          -          |       -        |  36.5  |    [model](https://s3.ap-northeast-2.amazonaws.com/open-mmlab/mmdetection/models/retinanet_r50_fpn_2x_20181125-8b724df2.pth)     |
+|    R-50-FPN     | pytorch |   2x    |    -     |          -          |       -        |  36.4  |    [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/retinanet_r50_fpn_2x_20190616-75574209.pth)     |
 |    R-101-FPN    |  caffe  |   1x    |   5.3    |        0.410        |      10.4      |  37.8  |                                                                -                                                                 |
 |    R-101-FPN    | pytorch |   1x    |   5.5    |        0.429        |      10.9      |  37.7  |    [model](https://s3.ap-northeast-2.amazonaws.com/open-mmlab/mmdetection/models/retinanet_r101_fpn_1x_20181129-f016f384.pth)    |
 |    R-101-FPN    | pytorch |   2x    |    -     |          -          |       -        |  38.1  |    [model](https://s3.ap-northeast-2.amazonaws.com/open-mmlab/mmdetection/models/retinanet_r101_fpn_2x_20181129-72c14526.pth)    |