Merge pull request #3207 from Gao-Jun/default_preview_port

Jekyll's default service port is 4000 instead of 9292 now

Author: JuanitoFatas

README.md

@@ -66,7 +66,7 @@ Then start a local web server with
 bundle exec rake serve
 ```
 
-Open [http://localhost:9292/](http://localhost:9292/)
+Open [http://localhost:4000/](http://localhost:4000/)
 in your browser to access the preview.
 
 **Note:** The build of the site will take several minutes.