manpage: update URL links to HOWTO/README

Web frontend of the official git repository seems to have been changed
after 9040e236 (manpage: Add URL links to HOWTO/README).

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
1 file changed