chore: update variable description (#41)

Author: luke-taylor

README.md

@@ -396,7 +396,7 @@ Default: `"ErGw1AZ"`
 
 ### <a name="input_subnet_address_prefix"></a> [subnet\_address\_prefix](#input\_subnet\_address\_prefix)
 
-Description: The address prefix for the gateway subnet. Either subnet\_id or subnet\_address\_prefix must be specified.
+Description: The address prefix for the gateway subnet. Required if `subnet_creation_enabled = true`.
 
 Type: `string`
 

variables.tf

@@ -300,7 +300,7 @@ variable "sku" {
 variable "subnet_address_prefix" {
   type        = string
   default     = ""
-  description = "The address prefix for the gateway subnet. Either subnet_id or subnet_address_prefix must be specified."
+  description = "The address prefix for the gateway subnet. Required if `subnet_creation_enabled = true`."
   nullable    = false
 }