Update chain.py

typo

Author: mzcu

chain.py

@@ -12,7 +12,7 @@ def __init__(self):
     def successor(self, successor):
         self._successor = successor
 
-    def handler(self, request):
+    def handle(self, request):
         raise NotImplementedError('Must provide implementation in subclass.')
 
 class ConcreteHandler1(Handler):