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

**Steps to reproduce:**
Create file `config.ru` in app root with the following content. (http://guides.rubyonrails.org/v2.3.11/rails_on_rack.html#rackup)

    require "config/environment"
    use Rails::Rack::LogTailer
    use Rails::Rack::Static
    run ActionController::Dispatcher.new

Run rackup using puma the web server.

    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:**
Run with `rackup -s Puma`, **NOT** `rackup -s puma`