TCP RACK: fix TCP_RACK_PACING_BETA socket option

Bring back the code, which was accidentally removed. While there,
indent a comment correctly.

Reviewed by: rrs
CID: 1540026
Fixes: e18b97bd63a8 ("Update to bring the rack stack with all its fixes in.")
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48340