archivers/py-borgbackup: fix pkg-message instructions

which were suggesting to add the loader.conf variable fusefs_load
to rc.conf instead via sysrc.

Reported by: zero1@zaclys.net
PR: 278429
MFH: 2024Q2

(cherry picked from commit 334e8e98b3a625df06b3a5c3d44b0cbc01c77f31)