Fixed Rouge install instructions for Debian-based systems

[lgiordani]

I fixed the Rouge installation instructions in the manual as they weren't working on my Ubuntu. I can't check on a pure Debian but I double-checked the name of the package in the following archives

https://packages.debian.org/search?keywords=ruby-rouge https://packages.ubuntu.com/search?keywords=ruby-rouge&searchon=names&suite=all&section=all

Thanks for Asciidoctor, a great piece of software!

[mojavelinux]

Thanks! My understanding is that apt is not yet universal (compared to apt-get). Could you clarify which one is the recommended command?

[lgiordani]

You are right, I changed that without thinking. It looks like apt hasn't replaced apt-get, but is intended more as a visually pleasant wrapper (see http://manpages.ubuntu.com/manpages/hirsute/en/man8/apt.8.html). I'm not fussed about that, and since apt-get will continue working I'm going to revert that part of the change. Thanks!

[mojavelinux]

The old user manual has been replaced and I applied this issue to the new docs. See https://docs.asciidoctor.org/asciidoctor/latest/syntax-highlighting/rouge/#install-rouge