[fix] incorrect example of uploadURL (corrupting local file ID stored by uploader)

was not critical before 1.7 - became critical now, the updated Uploader can't change the status from "client" to "server" or "error" and the request hangs as if a pending one (spinning icon and the card is not updated) plus can't set the cover

Author: kullias

docs/guides/configuration.md

@@ -313,8 +313,7 @@ new kanban.Kanban("#root", {
                     .then(res => res.json())
                     .then(
                         data => {
-                            rec.id = data.id;
-                            return data;
+                            return { id: rec.id, ...data };
                         },
                         () => ({ id: rec.id, status: "error" })
                     )