fortune: use 115200 in serial example

Commit 4722ceb7d53e switched the default serial rate to 115200 bps.
Follow suit in the freebsd-tips fortune example.

Reviewed by: bcr
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41495

(cherry picked from commit aaff6a85ff1e506e38a8c4f94f1cb1065427e8c3)