Fix code example (#3334)

Add parentheses

Author: leoruhland

docs/auth.md

@@ -76,7 +76,7 @@ export class UserComponent implements OnDestroy {
   ...
 
   constructor() {
-    this.userSubscription = this.user$.subscribe(aUser: User | null => {
+    this.userSubscription = this.user$.subscribe((aUser: User | null) => {
         //handle user state changes here. Note, that user will be null if there is no currently logged in user.
      console.log(aUser);
     })