Updated network.html documentation to include brokenImage property

docs/network.html

@@ -964,6 +964,12 @@ All options defined per-node override these global settings.
     <td>undefined</td>
     <td>Default image url for the nodes. only applicable to shape <code>image</code>.</td>
   </tr>
+  <tr>
+    <td class="greenField">brokenImage</td>
+    <td>String</td>
+    <td>undefined</td>
+    <td>Image url to use in the event that the url specified in the <code>image</code> property fails to load. only applicable to shape <code>image</code>.</td>
+  </tr>
   <tr>
     <td class="greenField">mass</td>
     <td>number</td>