[Docs] Fix minor typo in docs/en/config.md (#1797)

* minor typo

minor typing bug. replaced `deprecated.py` to `deprecated_cfg.py`

* fixing the typo

as per the example, the directory name is `converters` not `converter`.

docs/en/understand_mmcv/config.md

@@ -196,5 +196,5 @@ _deprecation_ = dict(
 ```python
 >>> cfg = Config.fromfile('./deprecated_cfg.py')
 
-UserWarning: The config file deprecated.py will be deprecated in the future. Please use expected_cfg.py instead. More information can be found at https://github.com/open-mmlab/mmcv/pull/1275
+UserWarning: The config file deprecated_cfg.py will be deprecated in the future. Please use expected_cfg.py instead. More information can be found at https://github.com/open-mmlab/mmcv/pull/1275
 ```

docs/en/understand_mmcv/registry.md

@@ -31,7 +31,7 @@ In the package, we first create a file to implement builders, named `converters/
 ```python
 from mmcv.utils import Registry
 # create a registry for converters
-CONVERTERS = Registry('converter')
+CONVERTERS = Registry('converters')
 ```
 
 Then we can implement different converters in the package. For example, implement `Converter1` in `converters/converter1.py`