Simplify log file rotation example

The rfs module will automatically create the log directory (https://www.npmjs.com/package/rotating-file-stream#function-filenameindex) so this example doesn't need to manually check for or create the directory.

README.md

@@ -309,20 +309,18 @@ app.get('/', function (req, res) {
 Simple app that will log all requests in the Apache combined format to one log
 file per day in the `log/` directory using the
 [rotating-file-stream module](https://www.npmjs.com/package/rotating-file-stream).
+The `rotating-file-stream` module will automatically create the log directory in
+this example if it doesn't already exist.
 
 ```js
 var express = require('express')
-var fs = require('fs')
 var morgan = require('morgan')
 var path = require('path')
 var rfs = require('rotating-file-stream')
 
 var app = express()
 var logDirectory = path.join(__dirname, 'log')
 
-// ensure log directory exists
-fs.existsSync(logDirectory) || fs.mkdirSync(logDirectory)
-
 // create a rotating write stream
 var accessLogStream = rfs('access.log', {
   interval: '1d', // rotate daily