Fixed default value of option backgroundColor

docs/graph3d/index.html

@@ -298,7 +298,7 @@ var options = {
     <tr>
       <td>backgroundColor</td>
       <td>string or Object</td>
-      <td>'white'</td>
+      <td>{fill:&nbsp;'white', stroke:&nbsp;'gray', strokeWidth:&nbsp;1}</td>
       <td>The background color for the main area of the chart.
         Can be either a simple HTML color string, for example: 'red' or '#00cc00',
         or an object with the following properties.</td>