Fix error message to comply with staticcheck

[jjngx]

Proposed changes

This PR fixes staticcheck error: pkg/apis/dos/validation/dos.go:206:11: error strings should not be capitalized (ST1005)

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.23%. Comparing base (45d3f46) to head (d8e5870).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6421   +/-   ##
=======================================
  Coverage   53.23%   53.23%           
=======================================
  Files          85       85           
  Lines       16002    16002           
=======================================
  Hits         8518     8518           
  Misses       7079     7079           
  Partials      405      405           

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