Add dynu_record_import_id variable for record imports

infrastructure/terraform/dynu/imports.tf.example

@@ -12,5 +12,5 @@ import {
 #
 # import {
 #   to = dynu_dns_record.grafana_lan_ddnsgeek_com
-#   id = "REPLACE_WITH_DYNU_RECORD_IMPORT_ID"
+#   id = var.dynu_record_import_id
 # }