Make the tap device name the same across all the examples to
not confuse people. Of course, when doing these examples from
top to bottom, the FreeBSD bhyve VM will use tap0, the next
tap1, etc., but people might not want to stick to that.
Besides, the tap device definition is enclosed in replaceable
tags, so that people know that they need to put their local
device names there and not just copy-and-paste examples without
thinking about them.

Submitted by: rodrigo
Differential Revision: https://reviews.freebsd.org/D9560