Fix AES69 reference link

[JSap0914]

Closes #139

Changes proposed in this pull request:

• Replace the dead AES standards search URL in the README reference with the live SOFA conventions page.
• Keep the AES69-2022 bibliographic citation intact while pointing readers to public SOFA/AES69 revision information.

Validation

• git diff --check
• curl -L https://www.aes.org/publications/standards/search.cfm?docID=99 -> 404
• curl -L https://www.sofaconventions.org/mediawiki/index.php/SOFA_(Spatially_Oriented_Format_for_Acoustics) -> 200
• UV_PROJECT_ENVIRONMENT=../.venv uv run --project .. --extra docs --extra tests python -m sphinx -b html . _build/html from docs/ -> passed
• python -m sphinx -b linkcheck . _build/linkcheck confirmed the new SOFA conventions URL as ok; the command exits nonzero because of pre-existing unrelated broken links in the docs (verification_rules, an old MIDI URL, and generated relative links).

Duplicate check

Immediately before opening this PR, searches for aes.org, docID=99, AES69 link, AES69 reference, and dead link in pyfar/sofar PRs returned no matching open or closed PRs for this README reference fix.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.