Corrections for S3 deployment documentation

Hi Werckers!

This pull request contains three small corrections for the S3 deployment documentation:

* The `s3sync` step needs a colon to be considered valid YAML.
* The `s3sync` step's option is `source_dir`, not `source`.
* The screenshot defines a `$URL` variable while the example uses a `$BUCKET` variable. I changed the example to use a `$URL` variable.

Hopefully you can use the changes in this pull request. Thanks for the cool product!