ReactWrapper#update example is incorrect

The code example for ReactWrapper's update method (https://github.com/enzymejs/enzyme/blob/master/docs/api/ReactWrapper/update.md#example) is incorrect.

I'll reproduce it here:

class ImpureRender extends React.Component {
  constructor(props) {
    super(props);
    this.count = 0;
  }

  render() {
    this.count += 1;
    return <div>{this.count}</div>;
  }
}

const wrapper = mount(<ImpureRender />);
expect(wrapper.text()).to.equal('0');
wrapper.update();
expect(wrapper.text()).to.equal('1');

The first issue is that the initial render output is "1" not "0" because the counter is incremented on the first render before this.value is used in the return value. The second issue is that incrementing the counter is exactly what doesn't happen when using wrapper.update() (unlike wrapper.setProps()).

I didn't just submit a PR to fix these two details because I think a bigger revision is needed to illustrate the difference between wrapper.update() and wrapper.setProps({}) more clearly, but I'm not sure what yet.