Change example value to lower case in ECS service (#12661)

According to the validator, the only acceptable values are 'cpu' and
'memory' (both lower case).
73f4acf041/builtin/providers/aws/validators.go (L722):L725
Updating the docs so that this example works out of the box.

website/source/docs/providers/aws/r/ecs_service.html.markdown

@@ -27,7 +27,7 @@ resource "aws_ecs_service" "mongo" {
 
   placement_strategy {
     type  = "binpack"
-    field = "CPU"
+    field = "cpu"
   }
 
   load_balancer {