Change to documentation for AWS AMI request

Hi guys, it just might be me, but clicking the AMI from http://cloud-images.ubuntu.com/locator/ec2/ is broken to me. So I just did it via the normal Create Instance Wizard.

I just though some people might have the same issue.

Sorry if my markdown for links is screwed up, I went by the examples, since I'm used to the `[]()` traditional one.

Cheers.
Merge pull request #2772 from bitoiu/patch-3

Change to documentation for AWS AMI request