add comment on OPP example

Hamidalavi · Hamidalavi · commit df7641326f51 · 2021-03-25T14:41:56.000+04:30
diff --git a/Learn/9. OOP.md b/Learn/9. OOP.md
@@ -99,6 +99,7 @@ Product {
 Consider:
 
 ```js
+// top property or field is in ES7
 class Product {
   category = "DEFAULT";
   constructor(title) {
