[Docs] Fix the example error in registry.md (#2243)

the lack of colon(:)

docs/zh_cn/understand_mmcv/registry.md

@@ -56,7 +56,7 @@ from .converter1 import Converter1
 
 # 使用注册器管理模块
 @CONVERTERS.register_module()
-def converter2(a, b)
+def converter2(a, b):
     return Converter1(a, b)
 ```