update ffmpeg ubuntu code example

Example 1: ubuntu 18 ffmpeg install

sudo apt install software-properties-common
sudo apt update
sudo add-apt-repository ppa:jonathonf/ffmpeg-4
sudo apt install ffmpeg
ffmpeg -version

Example 2: install ffmpeg ubuntu

# On Ubuntu 18.04

$ sudo add-apt-repository ppa:jonathonf/ffmpeg-4
$ sudo apt install ffmpeg

# check version
$ ffmpeg --version

# You can download Windows builds from ffmpeg site.