docs: Clarify wording of which variables prompt

website/source/intro/getting-started/variables.html.md

@@ -29,11 +29,11 @@ variable "region" {
 }
 ```
 
-This defines three variables within your Terraform configuration.
-The first two have empty blocks `{}`. The third sets a default. If
-a default value is set, the variable is optional. Otherwise, the
-variable is required. If you run `terraform plan` now, Terraform will
-prompt you for the values for these variables since the required variables are not set.
+This defines three variables within your Terraform configuration.  The first
+two have empty blocks `{}`. The third sets a default. If a default value is
+set, the variable is optional. Otherwise, the variable is required. If you run
+`terraform plan` now, Terraform will prompt you for the values for unset string
+variables.
 
 ## Using Variables in Configuration