disksim(8): add mbr to example
 [rsc] --rw-rw-r-- M 337084 rsc sys 1476 Apr  4 07:34 sys/man/8/disksim
	/n/sourcesdump/2006/0404/plan9/sys/man/8/disksim:64,69 - 
	/n/sourcesdump/2006/0405/plan9/sys/man/8/disksim:64,70
	  .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