disksim example missing disk/mbr which disk/fdisk requires.

Reference: /n/sources/patch/applied/disksim-man-mbr
Date: Tue Apr  4 00:32:50 CES 2006
Signed-off-by: steve@quintile.net

--- /sys/man/8/disksim	Tue Apr  4 00:32:24 2006
+++ /sys/man/8/disksim	Tue Apr  4 00:32:22 2006
@@ -64,6 +64,7 @@
 .EX
 aux/disksim
 echo geometry 40000 512 0 0 0 >/dev/sdXX/ctl # 20MB
+disk/mbr /dev/sdXX/data
 disk/fdisk -baw /dev/sdXX/data
 disk/prep /dev/sdXX/plan9
 .EE