Fix faster_rcnn_resnet50_coco model source link (#85)

Author: bhadur

docs/MODELS.md

@@ -22,7 +22,7 @@ The links to the TensorFlow-Slim models include the pre-trained checkpoint files
 |---|---|
 | [faster_rcnn_inception_resnet_v2_atrous_coco](http://download.tensorflow.org/models/object_detection/faster_rcnn_inception_resnet_v2_atrous_coco_2018_01_28.tar.gz)| CPU, iGPU, MYRIAD, VAD-M |
 | [faster_rcnn_inception_v2_coco](http://download.tensorflow.org/models/object_detection/faster_rcnn_inception_v2_coco_2018_01_28.tar.gz)| CPU, iGPU, MYRIAD, VAD-M |
-| [faster_rcnn_resnet50_coco](http://download.tensorflow.org/models/object_detection/faster_rcnn_resnet50_coco_2018_01_28.tar.gzs)| CPU, iGPU, MYRIAD, VAD-M |
+| [faster_rcnn_resnet50_coco](http://download.tensorflow.org/models/object_detection/faster_rcnn_resnet50_coco_2018_01_28.tar.gz)| CPU, iGPU, MYRIAD, VAD-M |
 | [faster_rcnn_resnet101_coco](http://download.tensorflow.org/models/object_detection/faster_rcnn_resnet101_coco_2018_01_28.tar.gz)| CPU, iGPU, MYRIAD, VAD-M |
 | [faster_rcnn_resnet50_lowproposals_coco](http://download.tensorflow.org/models/object_detection/faster_rcnn_resnet50_lowproposals_coco_2018_01_28.tar.gz)| CPU, iGPU, MYRIAD, VAD-M |
 | [ssd_inception_v2](http://download.tensorflow.org/models/object_detection/ssd_inception_v2_coco_2018_01_28.tar.gz)| CPU, iGPU, MYRIAD, VAD-M |