Fix an error when run apt-get update

I got an error after run an apt-get update recently, the error message look like these

W: Failed to fetch http://ppa.launchpad.net/jonls/redshift-ppa/ubuntu/dists/precise/main/source/Sources  404  Not Found

To fix it you can run the following command

gksu software-properties-gtk

and remove the problematic package (ppa.launchpad.net in this case), and then try apt-get update again.