Update _Apt-Repositories.rst (ros2#5867)

Use `$UBUNTU_CODENAME` if available

Signed-off-by: Yannis Gerlach <[email protected]>

Author: ygerlach

source/Installation/_Apt-Repositories.rst

@@ -16,5 +16,5 @@ Updates to repository configuration will occur automatically when new versions o
 
    $ sudo apt update && sudo apt install curl -y
    $ export ROS_APT_SOURCE_VERSION=$(curl -s https://api.github.com/repos/ros-infrastructure/ros-apt-source/releases/latest | grep -F "tag_name" | awk -F\" '{print $4}')
-   $ curl -L -o /tmp/ros2-apt-source.deb "https://github.com/ros-infrastructure/ros-apt-source/releases/download/${ROS_APT_SOURCE_VERSION}/ros2-apt-source_${ROS_APT_SOURCE_VERSION}.$(. /etc/os-release && echo $VERSION_CODENAME)_all.deb" # If using Ubuntu derivates use $UBUNTU_CODENAME
+   $ curl -L -o /tmp/ros2-apt-source.deb "https://github.com/ros-infrastructure/ros-apt-source/releases/download/${ROS_APT_SOURCE_VERSION}/ros2-apt-source_${ROS_APT_SOURCE_VERSION}.$(. /etc/os-release && echo ${UBUNTU_CODENAME:-${VERSION_CODENAME}})_all.deb"
    $ sudo dpkg -i /tmp/ros2-apt-source.deb