Corrected the get_started.md example inference (#4429)

It did not contain the checkpoint download step.

docs/get_started.md

@@ -188,15 +188,18 @@ PYTHONPATH="$(dirname $0)/..":$PYTHONPATH
 
 ## Verification
 
-To verify whether MMDetection and the required environment are installed correctly, we can run sample python codes to initialize a detector and inference a demo image:
+To verify whether MMDetection and the required environment are installed correctly, we can run sample Python code to initialize a detector and run inference a demo image:
 
 ```python
 from mmdet.apis import init_detector, inference_detector
 
 config_file = 'configs/faster_rcnn/faster_rcnn_r50_fpn_1x_coco.py'
+# download the checkpoint from model zoo and put it in `checkpoints/`
+# url: http://download.openmmlab.com/mmdetection/v2.0/faster_rcnn/faster_rcnn_r50_fpn_1x_coco/faster_rcnn_r50_fpn_1x_coco_20200130-047c8118.pth
+checkpoint_file = 'checkpoints/faster_rcnn_r50_fpn_1x_coco_20200130-047c8118.pth'
 device = 'cuda:0'
 # init a detector
-model = init_detector(config_file, device=device)
+model = init_detector(config_file, checkpoint_file, device=device)
 # inference the demo image
 inference_detector(model, 'demo/demo.jpg')
 ```