fix tutorials typo (#1844)

Author: suchot

docs/en/tutorials/customize_models.md

@@ -85,7 +85,7 @@ Here we show how to develop new components with an example of MobileNet.
 ```python
 import torch.nn as nn
 
-from ..registry import BACKBONES
+from ..builder import BACKBONES
 
 
 @BACKBONES.register_module

docs/zh_cn/tutorials/customize_models.md

@@ -83,7 +83,7 @@ MMSegmentation 里主要有2种组件：
 ```python
 import torch.nn as nn
 
-from ..registry import BACKBONES
+from ..builder import BACKBONES
 
 
 @BACKBONES.register_module