Change in CLI behavior

Just wanted to call out that the CLI prompts for values for unset variables instead of an error.  Guessing that was an enhancement somewhere along the line and just didn't get updated in the docs.

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

@@ -33,7 +33,7 @@ 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
-error since the required variables are not set.
+prompt you for the values for these variables since the required variables are not set.
 
 ## Using Variables in Configuration