handbook/jails: Correct vnet jail example

In an AMA on the Community Discord, it was revealed that this example
was copied from Antranig's blog incorrectly. Correct the syntax here.

Reported by: Antranig Vartanian <antranigv@freebsd.am>
Reported by: Niclas <buero28@icloud.com>
Reviewed by: bcr, mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D48972