Switch AWS-UpdateSSMAgent doc to use virtual-host pathing

[jeffj254]

The aws:updateSsmAgent action in SSM uses the deprecated path-style addressing to download the manifest JSON from s3. This style of addressing can't be supported in environments that use url filtering and has also been deprecated by AWS (https://aws.amazon.com/blogs/aws/amazon-s3-path-deprecation-plan-the-rest-of-the-story/)

Current old path style:
https://s3.{Region}.amazonaws.com/amazon-ssm-{Region}/ssm-agent-manifest.json

Improved virtual path style:
https://amazon-ssm-{Region}.s3.{Region}.amazonaws.com/ssm-agent-manifest.json

It would be very helpful if the aws:updateSsmAgent action were changed to use the newer virtual path style addressing for downloads. Currently we can't take advantage of the built-in auto-updating functionality in any environments that have internet URL filtering.