docs: supply correct manifest.webmanifest filename (angular#37146)

Previously, the docs incorrectly identified the generated manifest file as "manifest.json" when it is actually "manifest.webmanifest".
PR Close angular#37146

Author: jorroll

aio/content/guide/service-worker-getting-started.md

@@ -23,7 +23,7 @@ The above command completes the following actions:
 2. Enables service worker build support in the CLI.
 3. Imports and registers the service worker in the app module.
 4. Updates the `index.html` file:
-    * Includes a link to add the `manifest.json` file.
+    * Includes a link to add the `manifest.webmanifest` file.
     * Adds meta tags for `theme-color`.
 5. Installs icon files to support the installed Progressive Web App (PWA).
 6. Creates the service worker configuration file called [`ngsw-config.json`](/guide/service-worker-config), which specifies the caching behaviors and other settings.