Update settings.asciidoc

Changed "must bet set" to "must be set" under Discovery Settings.

docs/reference/migration/migrate_5_0/settings.asciidoc

@@ -228,7 +228,7 @@ name of the setting must be prefixed with `es.`.
 
 ==== Discovery Settings
 
-The `discovery.zen.minimum_master_node` must bet set for nodes that have
+The `discovery.zen.minimum_master_node` must be set for nodes that have
 `network.host`, `network.bind_host`, `network.publish_host`,
 `transport.host`, `transport.bind_host`, or `transport.publish_host`
 configuration options set. We see those nodes as in "production" mode