Update README.md

README.md

 * **Restrictions**
     * No use of any built-in data structures, (`Collection`, `List`, `Map`)
     * Operations should be [idempotent](https://stackoverflow.com/questions/1077412/what-is-an-idempotent-operation)
-        * The output of the methods should always be the same if the input is the same.
+        * The output of the methods should always be the same, regardless of how many times the method is called.