Fix node readme misleading information

[gwenall]

Precise that a firewall port must be opened, which has nothing to do with DNS.

Checklist:

• [ ] The code follows our code conventions.
• [ ] Important principle changes have been documented in the wiki.
• [ ] The new code is covered by unit tests with assertions.
• [ ] The behaviour node and UI have been manually tested and is as expected.
• [ ] The squash commit message follows our conventions.

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

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