Update the documentation referencing Plan 9 from User Space.

The page originally referenced to checkout Plan9 application and libraries
have been missing for quite some time and the development is carried out
in github and documented on this new site.
Signed-off-by: Juan Manuel Méndez Rey <vejeta@gmail.com>
Link: https://lore.kernel.org/r/20200426015250.GA35090@camelotSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/9p.rst

@@ -192,4 +192,4 @@ For more information on the Plan 9 Operating System check out
 http://plan9.bell-labs.com/plan9
 
 For information on Plan 9 from User Space (Plan 9 applications and libraries
-ported to Linux/BSD/OSX/etc) check out http://swtch.com/plan9
+ported to Linux/BSD/OSX/etc) check out https://9fans.github.io/plan9port/