Update social_sites in README

This changes the documentation to match the version in _config.yml
in jekyll#141.

README.md

@@ -148,16 +148,17 @@ If you don't want to display comments for a particular post you can disable them
 You can add links to the accounts you have on other sites, with respective icon, by adding one or more of the following options in your config:
 
 ```yaml
-twitter_username: jekyllrb
-github_username:  jekyll
-dribbble_username: jekyll
-facebook_username: jekyll
-flickr_username: jekyll
-instagram_username: jekyll
-linkedin_username: jekyll
-pinterest_username: jekyll
-youtube_username: jekyll
-googleplus_username: +jekyll
+social_sites:
+  twitter: jekyllrb
+  github_:  jekyll
+  dribbble: jekyll
+  facebook: jekyll
+  flickr: jekyll
+  instagram: jekyll
+  linkedin: jekyll
+  pinterest: jekyll
+  youtube: jekyll
+  googleplus: +jekyll
 rss: rss
 ```