fix group desc error

Author: huangminlinux

ios/Classes/JmessageFlutterPlugin.m

@@ -1423,7 +1423,7 @@ - (void)updateGroupInfo:(FlutterMethodCall*)call result:(FlutterResult)result {
 
     JMSGGroup *group = resultObject;
     NSString *newName = group.displayName;
-    NSString *newDesc = group.description;
+    NSString *newDesc = group.desc;
 
     if (param[@"newName"]) {
       newName = param[@"newName"];