docs: add missing return to example storage.service.ts (#31434)

While JavaScript does support implicit returns, it seems TypeScript will not infer the function return type from
the implicit return of the last statement (at least not in TS 3.4.3). So, when the `return` is missing from the
`get` function the implicit type of the function is `void`. So for the `get` function to be usable it needs to an
explicit return.

PR Close #31434

aio/content/examples/dependency-injection-in-action/src/app/storage.service.ts

@@ -16,7 +16,7 @@ export class BrowserStorageService {
   constructor(@Inject(BROWSER_STORAGE) public storage: Storage) {}
 
   get(key: string) {
-    this.storage.getItem(key);
+    return this.storage.getItem(key);
   }
 
   set(key: string, value: string) {