Add clear also to demo.

Author: RoopeHakulinen

demo/index.html

@@ -159,6 +159,10 @@ <h3><a id="method" class="anchor" href="#method" aria-hidden="true"><span
 Flash.dismiss(1);
 // Dismiss the flash with id of 1. Id is not the index of flash but instead a value returned by Flash.create()
                 </pre>
+                <pre class="prettyprint linenums">
+Flash.clear();
+// Dismisses all flashes shown.
+                </pre>
         </div>
 
         <div class="section">