Fix work-dir flag name in example (#2622)

Just fixes a small typo in the example.

docs/en/train.md

@@ -73,7 +73,7 @@ An example:
 ```shell
 # checkpoints and logs saved in WORK_DIR=work_dirs/pspnet_r50-d8_512x512_80k_ade20k/
 # If work_dir is not set, it will be generated automatically.
-sh tools/dist_train.sh configs/pspnet/pspnet_r50-d8_512x512_80k_ade20k.py 8 --work_dir work_dirs/pspnet_r50-d8_512x512_80k_ade20k/ --deterministic
+sh tools/dist_train.sh configs/pspnet/pspnet_r50-d8_512x512_80k_ade20k.py 8 --work-dir work_dirs/pspnet_r50-d8_512x512_80k_ade20k/ --deterministic
 ```
 
 **Note**: During training, checkpoints and logs are saved in the same folder structure as the config file under `work_dirs/`. Custom work directory is not recommended since evaluation scripts infer work directories from the config file name. If you want to save your weights somewhere else, please use symlink, for example: