Adjust line length in tcp_prr_partialack

Summary:
Wrap lines before column 80 in new prr code checked in recently.

No functional changes.

Reviewers: tuexen, rrs, jtl, mm, kbowling, transport

Reviewed By: tuexen, mm, transport

Subscribers: imp, melifaro

Differential Revision: https://reviews.freebsd.org/D28329