Fix variable name in example

Readme.md

@@ -19,7 +19,7 @@ var graphite = require('graphite');
 var client = graphite.createClient('plaintext://graphite.example.org:2003/');
 
 var metrics = {foo: 23};
-carbonClient.write(metrics, function(err) {
+client.write(metrics, function(err) {
   // if err is null, your data was sent to graphite!
 });
 ```