Remove unnecessary sed command from quickstart

[pawl]

I'm not seeing any mention of REPLACE_ME_WITH_YOUR_PUBLIC_IP in the docker-compose.yml, so it seems safe to remove this command from the quickstart guide:

# Replace the public IP placeholder in the docker-compose.yml
sed -i "s/REPLACE_ME_WITH_YOUR_PUBLIC_IP/$(curl -s ifconfig.me)/g" docker-compose.yml