Add instructions for unregistering SW into tutorial

[helenvholmes]

While running through the tutorial I ran into the issue where after I switched branches (master > code-lab) the tutorial continued running offline because Chrome still had the service worker registered in memory. Might be helpful to add some quick instructions for unregistering it if others run into that issue:

If you do not get an error screen this is because the Service Worker needs to be unregistered. Open up the Chrome devtools and under Sources > Workers, click the Unregister button next to sw.js. Refresh the page, and you should get the standard error.