tcp_rack.4: Update the SEE ALSO section

Correct the surname of one of authors of RFC 8985 and add RFC 5681 and
RFC 6937 (PRR: Proportional Rate Reduction for TCP).

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D44399

(cherry picked from commit e29be07861173f87b2dd46db1d0c7cbcf72d4ad0)