4.0.6 release notes: a typo

(cherry picked from commit 428399d)

Author: michaelklishin

release-notes/4.0.6.md

@@ -138,7 +138,7 @@ Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://
 
 #### Enhancements
 
- * `` is a new configuration setting that allows the backend to skip registration.
+ * `cluster_formation.registration.enabled` is a new configuration setting that allows the backend to skip registration.
 
    This is useful when Consul is used for peer discovery but a different tool such as Nomad
    is used to keep track of the services and their registration, unregistration.