Correct ServiceWorkerRegistration's update() Web IDL
As mentioned in https://github.com/w3c/ServiceWorker/issues/1472, https://w3c.github.io/ServiceWorker/#resolve-job-promise-algorithm is written to return Promise<ServiceWorkerRegistration>.
Also, according to https://wpt.fyi/results/service-workers/service-worker/update-result.https.html?label=experimental&label=master&aligned, all implementations are actually returning a ServiceWorkerRegistration.
