Update article.md

Adding 'object' after 'Property descriptor' -- even though it is said not far above that the 'property descriptor' returned by `Object.getOwnPropertyDescriptor` is an object, I think it wouldn't hurt to discretely include that info here (though of course it's also implied by the usage in the examples). Especially if someone is quickly scanning the article looking for `Object.defineProperty` and didn't just read the part above.