Dead link in models.md

The link https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2022-0-201208-I!!PDF-E.pdf in [vmaf](https://github.com/Netflix/vmaf/tree/master)/[resource](https://github.com/Netflix/vmaf/tree/master/resource)/[doc](https://github.com/Netflix/vmaf/tree/master/resource/doc)
/models.md now returns a 404.