Fix an error in docs for manifest requirements for usbDevices permission

chrome/common/extensions/docs/templates/articles/app_usb.html

 <h1>USB Devices</h1>
 
 <p>
 This document describes how to use the <a href="usb">USB API</a> to communicate
 with USB devices. Some devices are not accessible through the USB API
 (see the <a href="#caveats">Caveats</a> section below for details).
 Chrome Apps can also connect to <a href="serial">serial</a> and
 <a href="bluetooth">Bluetooth</a> devices.
 </p>
 
@@ skipping 31 matching lines @@
   You can use $(ref:usb.getDevices) to enumerate devices by their VID/PID
   pair.
 </p>
 <p>
   You must declare the VID/PID pairs for each type of device you want to use
   under the "usbDevices" permission in your app's manifest file, as shown in the
   example below:</p>
 
 <pre data-filename="manifest.json">
 "permissions": [
-  "usbDevices": [
-    {
-      "vendorId": 123,
-      "productId": 456
-    }
-  ]
+  {
+    "usbDevices": [
+      {
+        "vendorId": 123,
+        "productId": 456
+      }
+    ]
+  }
 ]
 </pre>
 
 <p class="note">Note that only decimal numbers are allowed in JSON format.
   You cannot use hexadecimal numbers in these fields.</p>
 
 <h2 id="finding_device">Finding a device</h2>
 
 <p>
   To determine whether one or more specific devices are connected to a user's
@@ skipping 597 matching lines @@
     owned by group "plugdev" and have group write permissions.
   </li>
 </ul>
 
 <p>Your app cannot do this automatically since this this procedure requires root
 access. We recommend that you provide instructions to end-users and link to the
 <a href="#caveats">Caveats</a> section on this page for an explanation.</p>
 
 <p>On Chrome OS, simply call $(ref:usb.requestAccess). The permission
   broker does this for you.</p>