zfs-destroy.8: Fix minor formatting typo

The warning at the end of the second example in the description section
was actually inside the options table. Move the El macro to match what
is done in the first section for improved readability.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Ziaee <ziaee@FreeBSD.org>
Closes #16962