update index of 2018-Fall/

To ensure that the files is readable/accessible to the public when re-uploading to S3 bucket, use, eg:
`aws s3 cp /Users/thc4/Github/fieldnotes-content/content/2018-Fall/index.html s3://fieldnotes.makingandknowing.org/2018-Fall/index.html --acl public-read`