[Readme] Update for rake run task

Author: fabiopelosin

README.md

@@ -25,7 +25,7 @@ Steps to setup:
 2. `$ cd guides.cocoapods.org`
 3. `$ rake bootstrap`
 4. `$ bundle exec rake generate:all`
-5. `$ bundle exec middleman server`
+5. `$ rake run`
 6. Open [localhost:4567](http://localhost:4567) in your browser. Changes will be processed automatically
 
 ### Deploying the latest version to GitHub Pages