Update confusing wording in byo-firehose terraform module

[rfairburn]

The variable description for fleet_iam_role_arn is confusing in the byo-firehose-target-account module.

variable "fleet_iam_role_arn" {
  type        = string
  description = "the arn of the fleet role that firehose will assume to write data to your bucket"
}

This implies Fleet needs access to the s3 bucket when in reality only access to write to Firehose is required. Update the description to reflect that.