Documentation/fs/9p: fix broken link

In b529c06 (Update the documentation referencing Plan 9 from User
Space., 2020-04-26), another instance of the link was left unfixed.
Fix that as well.

Signed-off-by: Tuomas Ahola <taahol@utu.fi>
Message-ID: <20250322153639.4917-1-taahol@utu.fi>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: taahol

Documentation/filesystems/9p.rst

@@ -40,7 +40,7 @@ For remote file server::
 
 	mount -t 9p 10.10.1.2 /mnt/9
 
-For Plan 9 From User Space applications (http://swtch.com/plan9)::
+For Plan 9 From User Space applications (https://9fans.github.io/plan9port/)::
 
 	mount -t 9p `namespace`/acme /mnt/9 -o trans=unix,uname=$USER