Fix grammar in the contributor guide

[Rohan5commit]

PR Description

Fix a small grammar issue in CONTRIBUTING.md by changing "need documentation" to "needs documentation".

Related issue: Fixes #5391

Guideline alignment: single-file docs change from master, aligned with CONTRIBUTING.md.
Validation/testing: not run; docs-only change.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[Copilot]

Pull request overview

This PR fixes a minor grammar issue in the contributor PR checklist within CONTRIBUTING.md, improving clarity in the project’s contribution guidelines.

Changes:

• Update checklist item wording from “need documentation” to “needs documentation”.

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