Fix instance exit notification

Fix improper notification on Factorio server exit causing plugins and
the instance tracking to fail.

lib/plugin.js

@@ -72,9 +72,9 @@ class BaseInstancePlugin {
 	async onStop() { }
 
 	/**
-	 * Called when the instance is shutting down
+	 * Called when the Factorio server exited
 	 */
-	async onExit() { }
+	onExit() { }
 
 	/**
 	 * Called when the Factorio outputs a line