autoselect a network on the random network chooser window.

EduNetworkBuilder/RandomPuzzleChooser.cs

@@ -64,6 +64,7 @@ namespace EduNetworkBuilder
             {
                 cbChosenNet.Items.Add(one);
             }
+            cbChosenNet.SelectedIndex = 2;
         }
 
         private void cbChosenNet_Leave(object sender, EventArgs e)