Example file does not match the correct syntax

According to the new version of the documentation and most recent version of RabbitMQ (https://www.rabbitmq.com/cluster-formation.html), the example file did not reflect the changes (autocluster => cluster_formation)

(cherry picked from commit 5b7615c)

Author: almorelle

docs/rabbitmq.conf.example

@@ -352,23 +352,23 @@
 ## Relevant doc guide: http://rabbitmq.com//cluster-formation.html
 ##
 
-# autocluster.peer_discovery_backend     = rabbit_peer_discovery_classic_config
+# cluster_formation.peer_discovery_backend     = rabbit_peer_discovery_classic_config
 #
-# autocluster.classic_config.nodes.node1 = rabbit1@hostname
-# autocluster.classic_config.nodes.node2 = rabbit2@hostname
-# autocluster.classic_config.nodes.node3 = rabbit3@hostname
-# autocluster.classic_config.nodes.node4 = rabbit4@hostname
+# cluster_formation.classic_config.nodes.1 = rabbit1@hostname
+# cluster_formation.classic_config.nodes.2 = rabbit2@hostname
+# cluster_formation.classic_config.nodes.3 = rabbit3@hostname
+# cluster_formation.classic_config.nodes.4 = rabbit4@hostname
 
 ## DNS-based peer discovery. This backend will list A records
 ## of the configured hostname and perform reverse lookups for
 ## the addresses returned.
 
-# autocluster.peer_discovery_backend = rabbit_peer_discovery_dns
-# autocluster.dns.hostname           = rabbitmq.discovery.mycompany.local
+# cluster_formation.peer_discovery_backend = rabbit_peer_discovery_dns
+# cluster_formation.dns.hostname = discovery.eng.example.local
 
 ## This node's type can be configured. If you are not sure
 ## what node type to use, always use 'disc'.
-# autocluster.node_type = disc
+# cluster_formation.node_type = disc
 
 ## Interval (in milliseconds) at which we send keepalive messages
 ## to other cluster members. Note that this is not the same thing