Correct the destroy example.  The -n argument is not needed (and is not
valid).

Reported by:	mwlucas
Reviewed by:	phk
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270722 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f