modules/aws/vpc/sg-elb: Fix name for TNC security group

In 251147e (TNC bootstrapping, 2018-02-13,
coreos/tectonic-installer#3053), the resource was renamed from 'ncg'
to 'tnc', but the name property wasn't updated to match.

modules/aws/vpc/sg-elb.tf

@@ -2,7 +2,7 @@ resource "aws_security_group" "tnc" {
   vpc_id = "${data.aws_vpc.cluster_vpc.id}"
 
   tags = "${merge(map(
-      "Name", "${var.cluster_name}_console_sg",
+      "Name", "${var.cluster_name}_tnc_sg",
       "kubernetes.io/cluster/${var.cluster_name}", "owned",
       "tectonicClusterID", "${var.cluster_id}"
     ), var.extra_tags)}"