docs(sample/31): fix wrong port number in README

[NilsMoller]

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: Documentation update

What is the current behavior?

Following the README leads you to the wrong gateway link.

What is the new behavior?

The link in the README is correct.

Does this PR introduce a breaking change?

• Yes
• No

Other information

This was a regression after this PR fixed a version problem with sample 31.

[coveralls]

Pull Request Test Coverage Report for Build b7780d28-b81b-41b2-9e2c-e1552d25dacf

• 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 92.864%

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Totals
Change from base Build df8ed325-8a47-4e91-8f33-ed2a1e33d5d1:	0.0%
Covered Lines:	6364
Relevant Lines:	6853

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💛 - Coveralls