docs: bastion is a required attribute in variable users. (#39)

Adding bastion as true as it follows with the rest of the example

examples/gcp/README.md

@@ -30,6 +30,7 @@ $ cat <<EOF > inception/users.auto.tfvars
 users = [
  {
  id = "user:<your-user-email"
+ bastion = true
  inception = true
  platform = true
  logs = true