chore: add missing facebook copyright header

packages/docusaurus-plugin-content-docs/src/__tests__/__fixtures__/sidebars/sidebars-first-level-not-category.js

@@ -1,3 +1,10 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
 module.exports = {
   docs: [
     {