Merge pull request #2 from supercaizehua/patch-1

Update `03-类的继承关系.py`. Fix bug.

Author: muziing

01-HelloWorld-基本结构/03-类的继承关系.py

@@ -5,7 +5,7 @@ def get_sub_classes(class_):
     """递归地显示某一类的所有子类"""
     for subclass in class_.__subclasses__():
         print(subclass)
-        if len(class_.__subclasses__()) > 0:
+        if len(subclass.__subclasses__()) > 0:
             get_sub_classes(subclass)