Correct ServiceWorkerRegistration's update() Web IDL

As mentioned in #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.

Author: yoshisatoyanagisawa

docs/index.bs

@@ -556,7 +556,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
         readonly attribute USVString scope;
         readonly attribute ServiceWorkerUpdateViaCache updateViaCache;
 
-        [NewObject] Promise<undefined> update();
+        [NewObject] Promise<ServiceWorkerRegistration> update();
         [NewObject] Promise<boolean> unregister();
 
         // event