Fix incorrect docs about `special` default.
Fix incorrect docs about `special` default.

`special` actually defaults to true - https://github.com/terraform-providers/terraform-provider-random/blob/4fa426a0abefc452001cc3c55026b7e89b447bce/random/resource_string.go#L31.
terraform-providers/terraform-provider-random,terraform-providers/terraform-provider-random,hashicorp/terraform-provider-random,hashicorp/terraform-provider-random