staging: lustre: uapi: update URL doc link in lustre_fid.h

staging: lustre: uapi: update URL doc link in lustre_fid.h

The wiki has been revived so update the URL about the FID
design in lustre_fid.h.

Signed-off-by: Ben Evans <bevans@cray.com>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6401
Reviewed-on: https://review.whamcloud.com/22712
Reviewed-on: https://review.whamcloud.com/24569
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>