Replace deprecated lang target in cascade examples

> WARN  deprecated: cascade.target.language was deprecated in Hugo v0.150.0 and will be removed in a future release. cascade.target.sites.matrix instead, see https://gohugo.io/content-management/front-matter/#target

We changed it here:
<https://gohugo.io/content-management/front-matter/#target>

But forgot to change it here:
<https://gohugo.io/configuration/cascade/#lang>
