Use correct variable for multiple images (#59)

Looks like in [MR-41](#41) we added a broken exapmle for deploying ecr with multiple repos. Also the test does not seem to cover this example. Do we want to test this example?

examples/multiple-repo/main.tf

@@ -8,5 +8,5 @@ module "ecr" {
   stage        = "dev"
   name         = "app"
   use_fullname = false
-  list_image   = ["redis", "nginx"]
+  image_names  = ["redis", "nginx"]
 }