Add variable type to pass the linter check

[YevheniiPokhvalii]

Please change terraform-example-module
/variables.tf from:

variable "example" {
  description = "Example variable"
  default     = "hello world"
}

to

variable "example" {
  type        = string
  description = "Example variable"
  default     = "hello world"
}

in order for the terraform lint to pass all checks.