Err occurred during the installation step on ubuntu

I have a ubuntu server 14.04 LTS.
I followed this instruction intend to install nginx.
When I use command

sudo apt-get update

I noticed the Err occurred like:

Err http://nginx.org trusty/nginx Sources
404 Not Found [IP: 206.251.255.63 80]
Err http://nginx.org trusty/nginx amd64 Packages
404 Not Found [IP: 206.251.255.63 80]
Err http://nginx.org trusty/nginx i386 Packages
404 Not Found [IP: 206.251.255.63 80]

Does anyone know why this would happen?