Fix mistaken reference to --skip-test-unit in README
Fix mistaken reference to --skip-test-unit in README

This comment in the README mentioned using the options --skip-test-unit and --skip-sprockets, but `rails/test_unit/railtie` isn't the one that's commented out. `action_mailer/railtie` is.

Signed-off-by: David Celis <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@davidcel.is>
djom20/rails-api,sideci-sample/sideci-sample-rails-api,rails-api/rails-api,ikhakoo/rails-api,GHolmes04/rails-api,PatrickEifler/rails-api,ryoconnell/rails-api,bjenkins100/rails-api,ipmobiletech/rails-api,joshbedo/rails-api,kyaukyuai/rails-api,cyberid41/rails-api