Modified sample to match updated service worker plugin
Modified sample to match updated service worker plugin

The preference for defining the service worker script changed from 'service_worker' to 'serviceworker'

MobileChromeApps/cordova-plugin-service-worker-background-sync,MobileChromeApps/cordova-plugin-service-worker-background-sync,MobileChromeApps/cordova-plugin-service-worker-background-sync