Fix style and comment around concave/convex regions in TCP cubic.

In cubic, the concave region is when snd_cwnd starts growing slower
towards max_cwnd (cwnd at the time of the congestion event), and
the convex region is when snd_cwnd starts to grow faster and
eventually appearing like slow-start like growth.

PR: 238478
Reviewed by: tuexen (mentor), rgrimes (mentor)
Approved by: tuexen (mentor), rgrimes (mentor)
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D24657