Add codeblock syntax highlighting

docs/custom-image.md

@@ -25,7 +25,8 @@ Before we can build the image, we have to set the k8s version we want to build t
 If you don't specify the version, the latest version, -2, will be used. If the latest is, e.g., 1.30, then 1.28 will be used.   
 
 For example for k8s version `1.28.3`:
-```
+
+```sh
 export PACKER_FLAGS="--var 'kubernetes_rpm_version=1.28.3' --var 'kubernetes_semver=v1.28.3' --var 'kubernetes_series=v1.28' --var 'kubernetes_deb_version=1.28.3-1.1'"
 ```