Add variable type to pass the linter check

Please change [terraform-example-module](https://github.com/cloudposse/terraform-example-module/tree/main)
/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.