docs: merge generator typo in example

I'm pretty sure the documented example for the merge generator has a slight typo/error in it.
Comment in code [here](https://argocd-applicationset.readthedocs.io/en/stable/Generators-Merge/#example-base-cluster-generator-override-cluster-generator-list-generator) should be:

> For clusters with a specific label, **disable** Kafka.

