Small change to readme

[chrislerum]

Under 'Basic Demo Setup', step 11 says go to localhost:3000, but when I set this up from scratch just now, it started the rails app at localhost:5000.

[justin808]

The puma.rb file says: https://github.com/shakacode/react-webpack-rails-tutorial/blob/master/config/puma.rb#L12

Port 3000.

I think you have something overriding it.