Update get_started.md (open-mmlab#1290)

fix wrong file format. it should be png instead of jpg

Author: Tsingularity

docs/en/get_started.md

@@ -237,7 +237,7 @@ python demo/image_demo.py ${IMAGE_FILE} ${CONFIG_FILE} ${CHECKPOINT_FILE} [--dev
 Examples:
 
 ```shell
-python demo/image_demo.py demo/demo.jpg configs/pspnet/pspnet_r50-d8_512x1024_40k_cityscapes.py \
+python demo/image_demo.py demo/demo.png configs/pspnet/pspnet_r50-d8_512x1024_40k_cityscapes.py \
     checkpoints/pspnet_r50-d8_512x1024_40k_cityscapes_20200605_003338-2966598c.pth --device cuda:0 --palette cityscapes
 ```