docs(events): update demo in usage

Closes #11428
Make it clear that event.subscribe must be called in another method

src/util/events.ts

@@ -12,19 +12,21 @@ import { ScrollView } from '../util/scroll-view';
  * ```ts
  * import { Events } from 'ionic-angular';
  *
- * constructor(public events: Events) {}
- *
  * // first page (publish an event when a user is created)
- * function createUser(user) {
+ * constructor(public events: Events) {}
+ * createUser(user) {
  *   console.log('User created!')
  *   events.publish('user:created', user, Date.now());
  * }
  *
- * // second page (listen for the user created event)
+ *
+ * // second page (listen for the user created event after function is called)
+ * constructor(public events: Events) {
  *   events.subscribe('user:created', (user, time) => {
  *     // user and time are the same arguments passed in `events.publish(user, time)`
  *     console.log('Welcome', user, 'at', time);
  *   });
+ * }
  *
  * ```
  * @demo /docs/demos/src/events/