[Extensions Docs] Fix comment style in css file

Bug: 521905
Change-Id: I7a2435279975c2f76961afe8176d4e4c0118f501
Reviewed-on: https://chromium-review.googlesource.com/1110659Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569533}

chrome/common/extensions/docs/examples/extensions/app_launcher/popup.css

-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
+/* Copyright (c) 2010 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file. */
 
 html {
   overflow-x: hidden;