swapon.2: correct FreeBSD release that introduced swapoff

It was introduced in 92da00bb245b, after 5.0 and beforen 5.1. Reported
in https://github.com/ziglang/zig/issues/16590.

(cherry picked from commit b15f6400376a90d3b00aa3ac00666f683f975376)
(cherry picked from commit f4bb052a789eae8ddcbfa7a9c00bc781b54e13c4)