rename the class name into powerpoint

OfficeJS/component/left_nav_bar.html

@@ -44,10 +44,10 @@
       New Spreadsheet
     </a>
   </li>
-  <li class="spreadsheet left-nav-tools">
+  <li class="powerpoint left-nav-tools">
     <a href="#/powerpoint"
        onclick="javascript:
-                OfficeJS.open({app:'powerpoint'});
+                OfficeJS.open({app:'power_point'});
                 return false;">
       <i class="icon-facetime-video"></i>
       New Powerpoint