Add "Downgrade from nightly to stable" instructions on Ubuntu Install page

[ElectricRCAircraftGuy]

I was using nightly and decided to downgrade to stable 5.0.0 now that it's out. It wasn't obvious to me how to do so. Please add downgrade instructions here: http://kicad-pcb.org/download/ubuntu/

Just under the "Nightly Development Builds" section:

Downgrade from Nightly to Stable:

sudo add-apt-repository --remove ppa:js-reynaud/ppa-kicad
sudo add-apt-repository --yes ppa:js-reynaud/kicad-5
sudo apt update
sudo apt install kicad

Done.

Originally posted here: https://bugs.launchpad.net/kicad/+bug/1783165