[Doc] Fix inference doc (open-mmlab#2787)

## Motivation

open-mmlab#2779

Author: xiexinch

docs/en/inference.md

@@ -56,9 +56,9 @@ Assume that you have already downloaded the checkpoints to the directory `checkp
 3. Test PSPNet on PASCAL VOC (without saving the test results) and evaluate the mIoU.
 
    ```shell
-   python tools/test.py configs/pspnet/pspnet_r50-d8_512x1024_20k_voc12aug.py \
-       checkpoints/pspnet_r50-d8_512x1024_20k_voc12aug_20200605_003338-c57ef100.pth \
-       --eval mAP
+   python tools/test.py configs/pspnet/pspnet_r50-d8_512x512_20k_voc12aug.py \
+       checkpoints/pspnet_r50-d8_512x512_20k_voc12aug_20200617_101958-ed5dfbd9.pth \
+       --eval mIoU
    ```
 
 4. Test PSPNet with 4 GPUs, and evaluate the standard mIoU and cityscapes metric.