docs: fix GitHub capitalization in security policy

[scosemicolon]

Fixes a small documentation capitalization issue in the security policy.\n\nNo code changes.

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Summary by cubic

Fixed the capitalization of “GitHub” in the security policy advisory link to match official branding. No code changes.

^{Written for commit e63a158. Summary will update on new commits. Review in cubic}

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[cubic-dev-ai]

No issues found across 1 file

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