docs: fix incident response security link

[ifer47]

Description

Fix the private reporting link in .github/INCIDENT_RESPONSE.md so it points to the repository-level SECURITY.md file.

Motivation & Context

The current ./SECURITY.md link resolves to .github/SECURITY.md, which does not exist. The same document already links to ../SECURITY.md earlier for the same policy, so this keeps the references consistent.

Fixes #42467.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the contributing guidelines.
• This change only updates documentation.
• I checked that the updated relative link points to the repository root SECURITY.md file.