doc: Sync AWS presigned URL example with S3 docs

examples/aws-presigned-url/main.js

@@ -33,7 +33,7 @@ uppy.use(AwsS3, {
       return {
         method: data.method,
         url: data.url,
-        fields: {}
+        fields: data.fields
       }
     })
   }

website/src/docs/aws-s3.md

@@ -152,7 +152,7 @@ uppy.use(AwsS3, {
       },
       body: JSON.stringify({
         filename: file.name,
-        type: file.type
+        contentType: file.type
       })
     }).then((response) => {
       // Parse the JSON response.