[docs] Update README metrics link

[Lovlace777]

Summary

• Updates the README CHAOSS metrics link to the current metrics and metrics models knowledge base page.
• Keeps the change limited to the incorrect README URL reported in broken link in README #317.
• Preserves the surrounding wording and link text.

Validation

• git diff --check passed.
• curl.exe -L -I returned HTTP 200 for https://chaoss.community/kb-metrics-and-metrics-models/.
• uv lock --check passed.

Notes

• Fixes broken link in README #317.
• Compatibility impact: none; documentation-only change.
• Signed commits: yes.