fix show_config bug

Author: bubbliiiing

yolo.py

@@ -71,7 +71,7 @@ def __init__(self, **kwargs):
         self.__dict__.update(self._defaults)
         for name, value in kwargs.items():
             setattr(self, name, value)
-            setattr(self._defaults, name, value)  
+            self._defaults[name] = value 
 
         #---------------------------------------------------#
         #   获得种类和先验框的数量