Add missing option in example.

Author: mycroft

sbin/mount_union/mount_union.8

@@ -1,4 +1,4 @@
-.\"	$NetBSD: mount_union.8,v 1.2 1995/03/18 14:58:22 cgd Exp $
+.\"	$NetBSD: mount_union.8,v 1.3 1995/05/30 09:09:39 mycroft Exp $
 .\"
 .\" Copyright (c) 1994
 .\" The Regents of the University of California.  All rights reserved.
@@ -150,7 +150,7 @@ and then only for lookups.
 The commands
 .Bd -literal -offset indent
 mount -t cd9660 -o ro /dev/cd0a /usr/src
-mount -t union -o /var/obj /usr/src
+mount -t union -o -b /var/obj /usr/src
 .Ed
 .Pp
 mount the CD-ROM drive