Fix dead link to NetBSD sysctl man page (#212)

Use the offical NetBSD man page URL https://man.netbsd.org

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

src/lib.rs

@@ -125,7 +125,7 @@
 //! [4]: https://developer.apple.com/documentation/security/1399291-secrandomcopybytes?language=objc
 //! [5]: https://www.freebsd.org/cgi/man.cgi?query=random&sektion=4
 //! [6]: https://man.openbsd.org/getentropy.2
-//! [7]: https://netbsd.gw.com/cgi-bin/man-cgi?sysctl+7+NetBSD-8.0
+//! [7]: https://man.netbsd.org/sysctl.7
 //! [8]: https://leaf.dragonflybsd.org/cgi/web-man?command=random&section=4
 //! [9]: https://docs.oracle.com/cd/E88353_01/html/E37841/getrandom-2.html
 //! [10]: https://docs.oracle.com/cd/E86824_01/html/E54777/random-7d.html