Incorrect method used in express-composition example

@achrinza y travaille déjà.

Depuis le 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

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### Additional information

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### Reproduction

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