Update Gentoo docs to reflect the package name change from app-emulation/lxc-docker to app-emulation/docker

We discussed this in today's #docker-meeting, especially in light of both the eventual optionality of LXC and the naming movements happening for Red Hat, and I've implemented it in my overlay so this is the documentation update to match.

I've also reformatted the file to 80 columns, as requested. :)