Fix instructions in the zfsboot manual page.

zfsloader(8) fails to probe a slice containing ZFS pool if its second sector
contains traces of BSD label (DISKMAGIC == 0x82564557).
Fix manual page to show working example erasing such traces.

PR:		226714
Approved by:	avg (mentor)
MFC after:	3 days

stand/i386/zfsboot/zfsboot.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 15, 2014
+.Dd March 27, 2018
 .Dt ZFSBOOT 8
 .Os
 .Sh NAME
@@ -99,9 +99,9 @@ can also be installed in an MBR slice:
 .Bd -literal -offset indent
 gpart create -s mbr ada0
 gpart add -t freebsd ada0
-gpart create -s BSD ada0s1
 gpart bootcode -b /boot/boot0 ada0
 gpart set -a active -i 1 ada0
+dd if=/dev/zero of=/dev/ada0s1 count=2
 dd if=/boot/zfsboot of=/dev/ada0s1 count=1
 dd if=/boot/zfsboot of=/dev/ada0s1 iseek=1 oseek=1024
 .Ed