adding functor example

lectures/21_hash_tables_II/README.md

@@ -22,6 +22,8 @@ my_return_type operator() ( /* my list of args */ );
 };
 ```
 
+- See and run this simple functor [example](functor.cpp).
+
 ## 21.2 Why are Functors Useful?
 
 - One example is the default 3rd argument for std::sort. We know that by default STL’s sort routines will use