Fix markdown (#5)

Author: osterman

README.md

@@ -4,6 +4,7 @@ Terraform module to easily provision an AWS CloudFront CDN with an S3 or custom
 
 ## Usage
 
+```hcl
 module "cdn" {
   source           = "git::https://github.com/cloudposse/terraform-aws-cloudfront-s3-cdn.git?ref=master"
   namespace        = "${var.namespace}"
@@ -12,6 +13,7 @@ module "cdn" {
   aliases          = "${var.hostname}"
   parent_zone_name = "${var.parent_zone_name}"
 }
+```
 
 ### Generating ACM Certificate