Fix typo in NGINX Plus error response struct

[dhurley]

Proposed changes

Fix typo in NGINX Plus error response struct.

Checklist

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

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.91%. Comparing base (b8b0995) to head (5864946).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1948      +/-   ##
==========================================
- Coverage   88.91%   88.91%   -0.01%     
==========================================
  Files         111      111              
  Lines       12426    12426              
==========================================
- Hits        11049    11048       -1     
- Misses       1375     1376       +1     
  Partials        2        2              

Files with missing lines	Coverage Δ
internal/nginx/nginx_plugin.go	80.97% <ø> (ø)

... and 1 file with indirect coverage changes

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