[Ch15] Update two threads for lambdas.

package ch15;

public class RunThreads implements Runnable {

public class RunThreads {

  public static void main(String[] args) {

    RunThreads runner = new RunThreads();

    Thread alpha = new Thread(runner);

    Thread beta = new Thread(runner);

    Thread alpha = new Thread(RunThreads::printThreadName);

    Thread beta = new Thread(RunThreads::printThreadName);

    alpha.setName("Alpha thread");

    beta.setName("Beta thread");

    alpha.start();

    beta.start();

  }

  public void run() {

  public static void printThreadName() {

    for (int i = 0; i < 25; i++) {

      String threadName = Thread.currentThread().getName();

      System.out.println(threadName + " is running");