[readme] Fixed typo in code example

README.md

@@ -79,7 +79,7 @@ Example:
 
 ```javascript
 im.convert(['kittens.jpg', '-resize', '25x120', 'kittens-small.jpg'], 
-function(err, metadata){
+function(err, stdout){
   if (err) throw err;
   console.log('stdout:', stdout);
 });