git repo on aurora.apache.org references old incubator repo which is 404

http://aurora.apache.org/documentation/latest/contributing/ says:

First things first, you’ll need the source! The Aurora source is available from Apache git:

git clone https://git-wip-us.apache.org/repos/asf/incubator-aurora

This repo no longer exists.

We should

• have it forward to the correct repo
• update the website to correct the repo url