Incorrect method used in express-composition example

@achrinza đang làm issue này rồi.

Từ ngày 22/1/2022.

### Describe the bug

The [documentation](https://loopback.io/doc/en/lb4/express-with-lb4-rest-tutorial.html) refers to method `pEvent` which is no longer used. Instead, we should use the `once` method that is used by the example

### Logs

_No response_

### Additional information

_No response_

### Reproduction

https://loopback.io/doc/en/lb4/express-with-lb4-rest-tutorial.html