Fix gh-pages JsonResponseHandler example

This is correct syntax for the master branch; it shouldn't go live until the changes currently on master are released.

index.html

@@ -107,7 +107,9 @@ $run->pushHandler($handler);
 // first in the stack, it will be executed before the error
 // page handler, and will have a chance to decide if anything
 // needs to be done.
-$run->pushHandler(new JsonResponseHandler);
+if (Whoops\isAjaxRequest()) {
+  $run->pushHandler(new JsonResponseHandler);
+}
 
 // Register the handler with PHP, and you're set!
 $run->register();