docs(sample): 20-cache unused interceptor

[krystxf]

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PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• [x] 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
• [x] Other... Please describe: Sample code

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Cache interceptor code in 20-cache sample was defined but not use

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 5e03e14d-a123-46b8-9959-d73197ebfef4

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 92.213%

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Totals
Change from base Build 607f026c-38fd-456a-8156-82c04b58e80e:	0.0%
Covered Lines:	6750
Relevant Lines:	7320

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💛 - Coveralls

[kamilmysliwiec]

This was expected; we left that example interceptor there just for demonstration purposes (in case someone needs to customize the default one)