Remove prereleases form Ionic (gazebosim#521)

Signed-off-by: Addisu Z. Taddese <[email protected]>

Author: azeey

ionic/install_ubuntu.md

@@ -17,7 +17,6 @@ Then install Gazebo Ionic:
 ```bash
 sudo curl https://packages.osrfoundation.org/gazebo.gpg --output /usr/share/keyrings/pkgs-osrf-archive-keyring.gpg
 echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/pkgs-osrf-archive-keyring.gpg] http://packages.osrfoundation.org/gazebo/ubuntu-stable $(lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/gazebo-stable.list > /dev/null
-echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/pkgs-osrf-archive-keyring.gpg] http://packages.osrfoundation.org/gazebo/ubuntu-prerelease $(lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/gazebo-prerelease.list > /dev/null
 sudo apt-get update
 sudo apt-get install gz-ionic
 ```