Add tip to the create Debian VM instructions

Summary: I had to start over the entire install process because I hit Tab -> Enter and it started installing despite Continue not being selected.

Test Plan: Read it / install a Debian VM with these instructions

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2770