fix indent for Extensions

~~~sh
$ padrino g project foo -c compass
~~~

before:
~~~ruby 
module Foo
  class App < Padrino::Application
    register Padrino::Rendering
    register Padrino::Mailer
    register Padrino::Helpers
  register CompassInitializer

end
~~~

after
~~~ruby
module Foo
  class App < Padrino::Application
    register Padrino::Rendering
    register Padrino::Mailer
    register Padrino::Helpers
    register CompassInitializer

end
~~~