add z and x to expert mode help text

expert.html

@@ -76,7 +76,7 @@ $().ready(function(){
   </form>
 </div>
 <div id="layoutControlPanel">
-  Use '>' to zoom in, '<' to zoom out, click to probe signals and drag to pan.
+  Use 'z' or '>' to zoom in, 'x' or '<' to zoom out, click to probe signals and drag to pan.
   <form id="updateShow"> Show:
   <input type="checkbox" name="1" id="updateShow1" onchange="updateShow(this.name,this.checked)" />(diffusion)
   <input type="checkbox" name="3" id="updateShow3" onchange="updateShow(this.name,this.checked)" />(grounded diffusion)