Update UPGRADING.md

UPGRADING.md

@@ -16,4 +16,4 @@ From 0.11 to 0.12
         $serializer->serialize($data, 'json', SerializationContext::create()->setVersion(1));
 
 - All visit??? methods of the VisitorInterface, now require a third argument, the Context; the context is for example
-  passed as an additional argument to handler, exclusion strategies, and also available in event listeners.
+  passed as an additional argument to handlers, exclusion strategies, and also available in event listeners.