message: Use `toHaveBeenCalledTimes` and `toHaveBeenLastCalledWith` in examples
  I think `.mock.calls.length` is kinda deprecated....
diff: @@ -34,8 +34,8 @@ test('waits 1 second before ending the game', () => {
  const timerGame = require('../timerGame');
  timerGame();
  - expect(setTimeout.mock.calls.length).toBe(1);
  - expect(setTimeout.mock.calls[0][1]).toBe(1000);
  + expect(setTimeout).toHaveBeenCalledTimes(1)
  + expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);
  });
  ```
  @@ -63,7 +63,7 @@ test('calls the callback after 1 second', () => {
  // Now our callback should have been called!
  expect(callback).toBeCalled();
  - expect(callback.mock.calls.length).toBe(1);
  + expect(callback).toHaveBeenCalledTimes(1)
  });
  ```
  @@ -110,8 +110,8 @@ describe('infiniteTimerGame', () => {
  // At this point in time, there should have been a single call to
  // setTimeout to schedule the end of the game in 1 second.
  - expect(setTimeout.mock.calls.length).toBe(1);
  - expect(setTimeout.mock.calls[0][1]).toBe(1000);
  + expect(setTimeout).toHaveBeenCalledTimes(1)
  + expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);
  // Fast forward and exhaust only currently pending timers
  // (but not any new timers that get created during that process)
  @@ -122,8 +122,8 @@ describe('infiniteTimerGame', () => {
  // And it should have created a new timer to start the game over in
  // 10 seconds
  - expect(setTimeout.mock.calls.length).toBe(2);
  - expect(setTimeout.mock.calls[1][1]).toBe(10000);
  + expect(setTimeout).toHaveBeenCalledTimes(2)
  + expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 10000);
  });
  });
  ```
  @@ -170,7 +170,7 @@ it('calls the callback after 1 second via advanceTimersByTime', () => {
  // Now our callback should have been called!
  expect(callback).toBeCalled();
  - expect(callback.mock.calls.length).toBe(1);
  + expect(callback).toHaveBeenCalledTimes(1)
  });
  ```
  