fix: update the custom microservices example to complete the callback

[mrsimonemms]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] 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?

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

What is the current behavior?

Issue Number: N/A

The example in the docs never resolves due to the isDisposed boolean not being set.

I can't find any concrete documentation on this, but it appears that it's required to tell the publish method to resolve. Without it, the example runs the initial console.log and returns the packet data to the Observable, but the teardown never happens.

What is the new behavior?

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information