Fix cargo clippy issue

src/broadcast_messages.rs

@@ -42,5 +42,5 @@ pub enum BroadcastMessage {
     /// Stop a client with a given client ID
     StopSyncClient(SyncClientID),
     /// Propagate a new sync event
-    SyncEvent(UserID, SyncEvent),
+    SyncEvent(UserID, Box<SyncEvent>),
 }