fix(microservices): listen method is void return type

[kaznovac]

state correct return type (void) on the NestMicroservice.listen method

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

return type is misleading this is not a breaking-change

Other information

[coveralls]

Pull Request Test Coverage Report for Build 1e3b132f-bd5e-47cb-ba9e-28a894d906e2

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.651%

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Totals
Change from base Build 23fac820-c861-40be-bf3c-a307cbe03615:	0.0%
Covered Lines:	5008
Relevant Lines:	5291

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💛 - Coveralls