PEP 484: Add missing def to NewType example function (#2890)

pep-0484.txt

@@ -1558,7 +1558,7 @@ errors by creating simple classes. For example::
   class UserId(int):
       pass
 
-  get_by_user_id(user_id: UserId):
+  def get_by_user_id(user_id: UserId):
       ...
 
 However, this approach introduces a runtime overhead. To avoid this,