Need comment referencing /cloudwatch_to_splunk prefix in terraform-aws-cloudwatch-log-group

[JonRoma]

Add comment to ssm_prefix variable definition to refer to terraform-aws-cloudwatch-log-group module that defines same variable, in order that both modules use default values consistent with one another. See /pull/7.

variable "ssm_prefix" {
  description = "Prefix string to be applied to look up runtime SSM variables)"
  default     = "/cloudwatch_to_splunk"
}