chore: add use case for selecting dynamic modules to 18-context sample

[micalevisk]

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?

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

Does this PR introduce a breaking change?

• Yes
• No

Other information

From time to time people seem to not understand on how to use app.select over dynamic modules. I'll prepare another PR to improve the docs as well

[coveralls]

Pull Request Test Coverage Report for Build 83077029-578f-4069-ae6e-df9d7f883a48

Details

• 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 92.207%

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Totals
Change from base Build 0f9b0016-1853-4775-8f4d-a4d89fc35f0f:	0.0%
Covered Lines:	6744
Relevant Lines:	7314

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💛 - Coveralls