Address warning: mismatched indentations at 'end' with 'class' at 3

This commit addresses the `warning: mismatched indentations at 'end' with 'class' at 3`

```ruby
$ bundle exec rake test
/home/yahonda/src/github.com/rails/tailwindcss-rails/lib/tailwindcss/engines.rb:21: warning: mismatched indentations at 'end' with 'class' at 3
Run options: --seed 14561

........................

Finished in 0.031471s, 762.6091 runs/s, 6164.4235 assertions/s.
24 runs, 194 assertions, 0 failures, 0 errors, 0 skips
```

Author: yahonda

lib/tailwindcss/engines.rb

@@ -1,6 +1,6 @@
 module Tailwindcss
   module Engines
-     class << self
+    class << self
       def bundle
         FileUtils.mkdir_p(Rails.root.join("app/assets/builds/tailwind"))
         Rails::Engine.subclasses.select do |engine|