Remove link to serviceworke.rs (#849)

According to mdn/serviceworker-cookbook#335,  "The domain https://serviceworke.rs/ (do not open) has been overtaken and are used to steal clicks. This removes the domain and uses a dummy domain."

Author: emhoracek

README.md

@@ -574,13 +574,6 @@ object will contain:
 </tbody>
 </table>
 
-# Help
-
-**Service Worker Cookbook**
-
-The [Service Worker Cookbook](https://serviceworke.rs/) is full of Web Push
-examples using this library.
-
 # Running tests
 
 > Prerequisites: