fix config PREFIX in tmux man page

[jda_tapodi.net]

Summary

tmux man page contains a placeholder for config PREFIX that is not
replaced by port.

Test Plan

Looked a man page in work dir, it has correct path instead of
placeholder.

Before:

-f file       Specify an alternative configuration file.  By default,
              tmux loads the system configuration file from
              @SYSCONFDIR@/tmux.conf, if present, then looks for a user
              configuration file at ~/.tmux.conf.

After:

-f file       Specify an alternative configuration file.  By default, 
              tmux loads the system configuration file from 
              /usr/local/etc/tmux.conf, if present, then looks for a user 
              configuration file at ~/.tmux.conf.

[mat]

This looks ok, can you open a PR with the patch so I don't forget about it.

[jda_tapodi.net]

Done: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=223907