Corrections to the Testing UIs section

[jcallin]

1. We are told to add
   import "./components"; import "./smoke-screens";
   To our index.js. Neither of these files exist yet.

2. At the end of the "shallow mounting" section, the webpage refers to a componentDidUnmount method, which does not exist in React.

[mayteio]

I believe it's a typo and they meant componentWillUnmount.