Fix typo of error message

Author: thuydx55

Diff for: cocos2d/core/CCActionManager.js

@@ -116,7 +116,7 @@ cc.ActionManager = cc.Class.extend(/** @lends cc.ActionManager# */{
         if(!action)
             throw new Error("cc.ActionManager.addAction(): action must be non-null");
         if(!target)
-            throw new Error("cc.ActionManager.addAction(): action must be non-null");
+            throw new Error("cc.ActionManager.addAction(): target must be non-null");
 
         //check if the action target already exists
         var element = this._hashTargets[target.__instanceId];