Edit rackup start command to rackup -s Puma.

Steps to reproduce:
1. Create config.ru. (http://guides.rubyonrails.org/v2.3.11/rails_on_rack.html#rackup)
2. rackup -s puma

Expected Result:
Application runs with puma as it's web server.

Actual Result:
.bundle/gems/rack-1.1.6/lib/rack/handler.rb:21:in `const_get': wrong constant name puma (NameError)

Solution:
Start with rackup -s Puma, NOT rackup -s puma.
Merge pull request #243 from hendrauzia/patch-1

Edit rackup start command on readme.md to rackup -s Puma.