fix command manager event listener (#41)

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Co-authored-by: Josh Taylor <me@broccol.ai>

Author: powercasgamer

docs/discord/jda5.md

@@ -35,7 +35,7 @@ You then register the command manager as a JDA event listener. Example:
 
 ```java
 JDABuilder.createDefault(yourToken)
-    .addEventListeners(commandManager)
+    .addEventListeners(commandManager.createListener())
     .build();
 ```