Add demo flag documentation in error message

[nitisht]

Current error message is like

error: The following required arguments were not provided:
  --username <username>
  --password <password>
  --s3-endpoint-url <url>
  --s3-access-key-id <access-key>
  --s3-secret-key <secret-key>
  --s3-region <region>
  --s3-bucket-name <bucket-name>

Usage: parseable --username <username> --password <password> --s3-endpoint-url <url> --s3-access-key-id <access-key> --s3-secret-key <secret-key> --s3-region <region> --s3-bucket-name <bucket-name>

For more information try '--help'

We need to add the --demo flag option here itself. This way users know if they want they can easily experience parseable with demo mode without the needs to set up a bucket. This is how the message can look

You can also use the --demo flag to run Parseable with default object storage. For testing purposes only

[sakti]

Hi @nitisht, I will try to resolve this issue.