message [nfc]: Fix a reference in a comment

Thanks to Greg for pointing this out:
  zulip#1912 (comment)

Author: chrisbobbe

lib/model/message.dart

@@ -504,7 +504,7 @@ class MessageStoreImpl extends HasChannelStore with MessageStore, _OutboxMessage
   void handleMessageEvent(MessageEvent event) {
     // If the message is one we already know about (from a fetch),
     // clobber it with the one from the event system.
-    // See [fetchedMessages] for reasoning.
+    // See [reconcileMessages] for reasoning.
     messages[event.message.id] = event.message;
 
     _handleMessageEventOutbox(event);