document -B option

Notes:
Thu Jan 24 16:15:20 EST 2013 geoff
    done with shorter usage message.


Reference: /n/sources/patch/applied/mk9660-usage
Date: Thu Jan 24 03:57:18 CET 2013
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/src/cmd/disk/9660/dump9660.c	Thu Jan 24 03:57:12 2013
+++ /sys/src/cmd/disk/9660/dump9660.c	Thu Jan 24 03:57:10 2013
@@ -23,7 +23,7 @@
 usage(void)
 {
 	if(mk9660)
-		fprint(2, "usage: disk/mk9660 [-D:] [-9cjr] [-b bootfile] [-o offset blocksize] [-p proto] [-s src] cdimage\n");
+		fprint(2, "usage: disk/mk9660 [-D:] [-9cjr] [-b bootfile] [-B bootfile] [-o offset blocksize] [-p proto] [-s src] cdimage\n");
 	else
 		fprint(2, "usage: disk/dump9660 [-D:] [-9cjr] [-m maxsize] [-n now] [-p proto] [-s src] cdimage\n");
 	exits("usage");