Amazon Instance Builder Docs Still Reference Deprecated S3Endpoint

Per https://github.com/mitchellh/packer/commit/c791216 the `ec2-upload-bundle` command parameters changed from using `--url {{.S3Endpoint}}` to `--region {{.Region}}`

The docs at http://www.packer.io/docs/builders/amazon-instance.html still reference the `S3Endpoint` syntax which results in an error if used in a custom bundle upload command.  

```
    amazon-instance: ec2-bundle-vol complete.
==> amazon-instance: Error processing bundle upload command: template: tpl30:1:194: executing "tpl30" at <.S3Endpoint>: S3Endpoint is not a field of struct type instance.uploadCmdData
```

Would you please update the Amazon instance docs to help future folks avoid this?

The use of custom bundle commands is necessary due to the default sudo `PATH` environment per https://groups.google.com/forum/#!topic/packer-tool/69wHimRb964 and my current lack of will to fix our base AMI to handle this issue more gracefully.