Fix error in documentation of AWS S3 Multipart::prepareUploadPart(file, partData)

Fixes #1130. Thank you @mattes3!

website/src/docs/aws-s3-multipart.md

@@ -91,7 +91,7 @@ Return a Promise for an object with keys:
      UploadId: partData.uploadId,
      PartNumber: partData.number,
      Body: '', // Empty, because it is uploaded later
-     Expires: Date.now() + 5 * 60 * 1000
+     Expires: 5 * 60,
    }, (err, url) => { /* there's the url! */ })
    ```