[Ch15] Updated two threads example to use a fixed thread pool.

package ch15;

public class RunThreads {

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class RunJobs {

  public static void main(String[] args) {

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

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

    alpha.setName("Alpha thread");

    beta.setName("Beta thread");

    alpha.start();

    beta.start();

    ExecutorService pool = Executors.newFixedThreadPool(2);

    pool.execute(RunJobs::printThreadName);

    pool.execute(RunJobs::printThreadName);

    pool.shutdownNow();

  }

  public static void printThreadName() {