Fix small error in code in comment for PostMessage usage. No functional change, just trying to fix …

ppapi/cpp/instance.h

Index: ppapi/cpp/instance.h
===================================================================
--- ppapi/cpp/instance.h	(revision 134008)
+++ ppapi/cpp/instance.h	(working copy)
@@ -419,7 +419,7 @@
   ///           type="application/x-ppapi-postMessage-example"/>
   ///   <script type="text/javascript">
   ///     var plugin = document.getElementById('plugin');
-  ///     plugin.AddEventListener("message",
+  ///     plugin.addEventListener("message",
   ///                             function(message) { alert(message.data); },
   ///                             false);
   ///   </script>