Fix terragrunt README quick-start import to /v2

Author: james00012

modules/terragrunt/README.md

@@ -31,7 +31,7 @@ options := &terragrunt.Options{
 ```go
 import (
     "testing"
-    "github.com/gruntwork-io/terratest/modules/terragrunt"
+    "github.com/gruntwork-io/terratest/modules/terragrunt/v2"
     "github.com/stretchr/testify/assert"
 )