Added reference link to mdn docs for Object.keys(user)

Author: akxavier

1-js/04-object-basics/08-symbol/article.md

@@ -161,7 +161,7 @@ for (let key in user) alert(key); // name, age (no symbols)
 alert( "Direct: " + user[id] );
 ```
 
-`Object.keys(user)` also ignores them. That's a part of the general "hiding symbolic properties" principle. If another script or a library loops over our object, it won't unexpectedly access a symbolic property.
+[Object.keys(user)](mdn:js/Object/keys) also ignores them. That's a part of the general "hiding symbolic properties" principle. If another script or a library loops over our object, it won't unexpectedly access a symbolic property.
 
 In contrast, [Object.assign](mdn:js/Object/assign) copies both string and symbol properties: