Update outdated reference for Mozilla's ServiceWorker cookbooks (#360)

Update outdated reference for Mozilla's ServiceWorker cookbooks to point to the main Github repo

Author: fkiriakos07

README.md

@@ -105,7 +105,7 @@ $report = $webPush->sendOneNotification(
 ### Full examples of Web Push implementations
 * An example with web-push-php: [Minishlink/web-push-php-example](https://github.com/Minishlink/web-push-php-example)
 * Matthew Gaunt's [Web Push Book](https://web-push-book.gauntface.com) - a must read
-* Mozilla's [ServiceWorker Cookbooks](https://serviceworke.rs/push-payload_index_doc.html) (don't mind the `server.js` file: it should be replaced by your PHP server code with this library)
+* Mozilla's [ServiceWorker Cookbooks](https://github.com/mdn/serviceworker-cookbook/blob/master/push-payload/README.md) (don't mind the `server.js` file: it should be replaced by your PHP server code with this library)
 * Google's [introduction to push notifications](https://developers.google.com/web/fundamentals/getting-started/push-notifications/) (as of 03-20-2016, it doesn't mention notifications with payload)
 * you may want to take a look at my own implementation: [sw.js](https://github.com/Minishlink/physbook/blob/9bfcc2bbf7311a5de4628eb8f3ae56b6c3e74067/web/service-worker.js) and [app.js](https://github.com/Minishlink/physbook/blob/02a0d5d7ca0d5d2cc6d308a3a9b81244c63b3f14/app/Resources/public/js/app.js)