fixes rendering with newer versions of cfndsl that validates referenc…

…es which breaks when inlining

s3-cloudfront.cfndsl.rb

@@ -8,7 +8,7 @@
         Effect: 'Allow',
         Action: 's3:GetObject',
         Resource: FnJoin('', [ 'arn:aws:s3:::', Ref("Bucket"), '/*']),
-        Principal: { CanonicalUser: FnGetAtt('s3bucketOriginAccessIdentity', 'S3CanonicalUserId') }
+        Principal: { CanonicalUser: { "Fn::GetAtt" => ['s3bucketOriginAccessIdentity', 'S3CanonicalUserId'] }}
       }
     ]
   }