Clarify registering and unregistering service workers in README.md

Following on from facebook#2715 this PR updates the README.md with updated instructions for opting out of PWA features.

packages/react-scripts/template/README.md

@@ -1581,16 +1581,17 @@ that your web app is reliably fast, even on a slow or unreliable network.
 ### Opting Out of Caching
 
 If you would prefer not to enable service workers prior to your initial
-production deployment, then remove the call to `serviceWorkerRegistration.register()`
+production deployment, then remove the call to `registerServiceWorker();`
 from [`src/index.js`](src/index.js).
 
 If you had previously enabled service workers in your production deployment and
 have decided that you would like to disable them for all your existing users,
-you can swap out the call to `serviceWorkerRegistration.register()` in
-[`src/index.js`](src/index.js) with a call to `serviceWorkerRegistration.unregister()`.
-After the user visits a page that has `serviceWorkerRegistration.unregister()`,
-the service worker will be uninstalled. Note that depending on how `/service-worker.js` is served,
-it may take up to 24 hours for the cache to be invalidated.
+you can swap out the call to `registerServiceWorker();` in [`src/index.js`](src/index.js)
+with a call to `unregister();` - you will need to ensure you have imported
+`unregister` by replacing `import registerServiceWorker from './registerServiceWorker';` with
+`import { unregister } from './registerServiceWorker';`. After the user visits a page that has
+`unregister();`, the service worker will be uninstalled. Note that depending on how
+`/service-worker.js` is served, it may take up to 24 hours for the cache to be invalidated.
 
 ### Offline-First Considerations