Fix an error in docs for manifest requirements for usbDevices permission

The "usbDevices" entry in permission list should be inside an object

Review URL: https://codereview.chromium.org/886353008

Cr-Commit-Position: refs/heads/master@{#314971}

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

@@ -49,12 +49,14 @@ Bluetooth</a> samples.
 
 <pre data-filename="manifest.json">
 "permissions": [
-  "usbDevices": [
-    {
-      "vendorId": 123,
-      "productId": 456
-    }
-  ]
+  {
+    "usbDevices": [
+      {
+        "vendorId": 123,
+        "productId": 456
+      }
+    ]
+  }
 ]
 </pre>