Add override keyword in README.md example

```Scala
def createPublisher(elements: Long): Publisher[Int] = ???
```

overrides:

```Java
/**
   * This is the main method you must implement in your test incarnation.
   * It must create a Publisher for a stream with exactly the given number of elements.
   * If `elements` is `Long.MAX_VALUE` the produced stream must be infinite.
   */
  public abstract Publisher<T> createPublisher(long elements);
```