Fixed a typo in the source.

import java.awt.GridLayout;

import java.awt.event.*;

import java.io.Serial;

import java.util.Date;

import java.util.Random;

import java.util.Timer;

import java.util.TimerTask;

   * Rate at which new "Tetrising" circles are being produced, in microseconds.

   * Production rate increases invesely proportional to the number of circle selections clicked so far.

   */

  static private long PRODUCT_CICLE_US = 10000000;

  static private long PRODUCT_CIRCLE_US = 10000000;

  /**

   * Rate at which new "Tetrising" circles descend on the board, in microseconds.

   * Production rate increases invesely proportional to the number of circle selections clicked so far.

    // start up the new ones

    produceCircleFuture = tetrisExec.scheduleAtFixedRate(new initDropCircles(), 0,

        PRODUCT_CICLE_US / numClicks, TimeUnit.MICROSECONDS);

        PRODUCT_CIRCLE_US / numClicks, TimeUnit.MICROSECONDS);

    dropCircleFuture = tetrisExec.scheduleAtFixedRate(new moveDropCircles(), 0, DROP_CIRCLE_US / numClicks,

        TimeUnit.MICROSECONDS);

  }