Fix acknowledgements in FAST to not reference PFWG

[jnurthen]

https://w3c.github.io/apa/fast/#framework-for-accessible-specification-of-technologies

Appendix A specifies "A.1 Participants in the PFWG at the time of publication" PFWG has not existed for quite some time.

[RealJoshue108]

Noted @jnurthen