Update callbacks.md

Author: varunmc

doc/gettingstarted/callbacks.md

@@ -38,7 +38,7 @@ var fs = require('fs'),
     Rx = require('rx');
 
 // Wrap fs.rename
-var rename = Rx.Observable.fromCallback(fs.rename);
+var rename = Rx.Observable.fromNodeCallback(fs.rename);
 
 // Rename file which returns no parameters except an error
 var source = rename('file1.txt', 'file2.txt');
@@ -106,4 +106,4 @@ getData().toNodeCallback(function (err, data) {
 	if (err) { throw err; }
 	// Do something with the data
 });
-```
+```