MFC r331630: 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)

_M .
M stand/i386/zfsboot/zfsboot.8