Install guide for "from a Unix/Linux System" has instructions for ifupdown instead of netplan

The page,

https://help.ubuntu.com/18.04/installation-guide/amd64/apds04.html

has the section,

> edit /etc/network/interfaces, /etc/resolv.conf, /etc/hostname and /etc/hosts.

with sample content for /etc/network/interfaces, however when I have performed all the commands preceding it, the contents of /etc/network/interfaces is:

from `cat /etc/network/interfaces`:

```
# ifupdown has been replaced by netplan(5) on this system. See
# /etc/netplan for current configuration.
# To re-enable ifupdown on this system, you can run:
# sudo apt install ifupdown
```

The documentation should be update for instructions for netplan.