docs(README): update user -> author for v12

README.md

@@ -147,7 +147,7 @@ const cardHtml = await damageRoller.render("Knockback roll");
 const chatData = {
   style: CONST.CHAT_MESSAGE_STYLES.OTHER,
   rolls: damageRoller.rawRolls(),
-  user: game.user._id,
+  author: game.user._id,
   content: cardHtml,
 };