Upgrade [GROUPING] reference to rfc7656

> [GROUPING]
> J. Lennox; K. Gross; S. Nandakumar; G. Salgueiro; B. Burman. IETF. RTP Grouping Taxonomy. 20 July 2015. Internet Draft (work in progress). URL: https://tools.ietf.org/html/draft-ietf-avtext-rtp-grouping-taxonomy

https://tools.ietf.org/html/rfc7656
