Update tf.vars.example

The variables are strings not lists

Author: juliette-cast

examples/eks/eks_cluster_gitops/tf.vars.example

@@ -4,6 +4,6 @@ aws_cluster_region  = "PLACEHOLDER"
 aws_cluster_name    = "PLACEHOLDER"
 subnets             = ["PLACEHOLDER1", "PLACEHOLDER2"]
 vpc_id                    = "PLACEHOLDER"
-cluster_security_group_id = ["PLACEHOLDER1"]
-node_security_group_id    = ["PLACEHOLDER1]
+cluster_security_group_id = "PLACEHOLDER1"
+node_security_group_id    = "PLACEHOLDER1"
 profile                   = "default" # default aws cli profile is used, override if needed.