update Plan9 URL

Since Bell Labs has died, the Plan9 project website only exists at a
mirror on 9p.io

src/overview/what_redox_is.md

@@ -31,7 +31,7 @@ It is written such that you do not need any prior knowledge in Rust and/or OS de
 
 [Rust]:  https://www.rust-lang.org  
 [POSIX]: https://en.wikipedia.org/wiki/POSIX
-[Plan9]: http://plan9.bell-labs.com/plan9/index.html
+[Plan9]: http://9p.io/plan9/index.html
 [Minix]: http://www.minix3.org/
 [Linux]: https://en.wikipedia.org/wiki/Linux
 [BSD]: http://www.bsd.org/