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 <me@davidcel.is>
Merge pull request #90 from davidcelis/patch-1

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