message: docker: Update README
  Update deprecated example as Ubuntu18.04 has been removed. Update example to Ubuntu 20.04
diff: @@ -16,7 +16,7 @@ To create an image :
  ```bash
  cd /whereever/MAVSDK/docker
  -docker build -t mavsdk/mavsdk-ubuntu-18.04 -f Dockerfile-Ubuntu-18.04 .
  +docker build -t mavsdk/mavsdk-ubuntu-20.04 -f Dockerfile-Ubuntu-20.04 .
  ```
  To use the image :
  