fixing an example in readme.md

Readme.md

@@ -46,7 +46,7 @@ console.log('<img src="' + canvas.toDataURL() + '" />');
  node-canvas adds `Image#src=Buffer` support, allowing you to read images from disc, redis, etc and apply them via `ctx.drawImage()`. Below we draw scaled down squid png by reading it from the disk with node's I/O.
 
 ```javascript
-fs.readFile(__dirname + '/images/squid.png', function(err){
+fs.readFile(__dirname + '/images/squid.png', function(err, squid){
   if (err) throw err;
   img = new Image;
   img.src = squid;