docs(queues): correct event listener example for BullMQ

[clawrence121]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

The existing documentation was not updated correctly for the newer BullMQ package, and does not cover all use cases provided by the new package.

Issue Number: N/A

What is the new behavior?

Docs updated to work with BullMQ.

This matches the documentation that is found in the official BullMQ docs:
https://docs.bullmq.io/guide/nestjs#processor
https://docs.bullmq.io/guide/nestjs/queue-events-listeners

As well as the code behaviour and comments:
https://github.com/nestjs/bull/blob/master/packages/bullmq/lib/decorators/on-worker-event.decorator.ts
https://github.com/nestjs/bull/blob/master/packages/bullmq/lib/decorators/on-queue-event.decorator.ts

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM

[kamilmysliwiec]

Thanks!