Remove unnecessary whitespace

guides/source/rails_on_rack.md

@@ -58,7 +58,7 @@ Here's how it loads the middlewares:
 ```ruby
 def middleware
   middlewares = []
-  middlewares << [Rails::Rack::Debugger]  if options[:debugger]
+  middlewares << [Rails::Rack::Debugger] if options[:debugger]
   middlewares << [::Rack::ContentLength]
   Hash.new(middlewares)
 end