srv(4), mount: update v9fs usage

The v9fs usage is documented on
https://www.kernel.org/doc/Documentation/filesystems/9p.txt

R=rsc
http://codereview.appspot.com/6445072

man/man4/srv.4

@@ -90,7 +90,7 @@ handling authentication:
 .IP
 .EX
 srv -a sources.cs.bell-labs.com sources
-sudo mount -t 9P -o user=$USER,uid=`id -u`,gid=`id -g`,proto=unix
+sudo mount -t 9p -o trans=unix,uname=$USER,dfltuid=`id -u`,dfltgid=`id -g`
            `namespace`/sources /n/sources
 .EE
 .SH SOURCE