apt update fails: E: The repository 'http://ppa.launchpad.net/elementary-os/stable/ubuntu disco Release' does not have a Release file.

[robert rottermann]

Err:13 http://ppa.launchpad.net/elementary-os/stable/ubuntu disco Release
   404  Not Found [IP: 91.189.95.83 80]
Hit:14 http://archive.ubuntu.com/ubuntu disco-updates InRelease
Hit:16 http://archive.ubuntu.com/ubuntu disco-backports InRelease
Hit:17 http://archive.ubuntu.com/ubuntu disco-security InRelease
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/elementary-os/stable/ubuntu disco 
Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore 
disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
root at chrissy:/etc/apt#