Docs: remove () typo from troubleshooting doc

[nginx-aoife]

Problem & Proposed changes

An extra set of () in this relref is causing a broken link here https://docs.nginx.com/nginx-gateway-fabric/how-to/monitoring/troubleshooting/#broken-header-error.

Closes #ISSUE n/a

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [ ] I have read the CONTRIBUTING doc
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have checked that all unit tests pass after adding my changes
• [ ] I have updated necessary documentation
• [ ] I have rebased my branch onto main
• [ ] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes, please add a brief note that summarizes the change.

NONE

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.86%. Comparing base (e05f4cf) to head (19fbabe). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2803 +/- ## ======================================= Coverage 89.86% 89.86% ======================================= Files 107 107 Lines 10997 10997 Branches 50 50 ======================================= Hits 9883 9883 Misses 1054 1054 Partials 60 60 ```

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[nginx-aoife]

If approved please Merge too.

[ADubhlaoich]

Enabled automerge: I will take responsibility for creating a cherry pick to the release branch once this is merged.