[ci skip] Fix fenced code block lang

guides/source/4_0_release_notes.md

@@ -181,7 +181,7 @@ Please refer to the [Changelog](https://github.com/rails/rails/blob/4-0-stable/a
 * `String#to_date` now raises `ArgumentError: invalid date` instead of `NoMethodError: undefined method 'div' for nil:NilClass`
   when given an invalid date. It is now the same as `Date.parse`, and it accepts more invalid dates than 3.x, such as:
 
-  ```
+  ```ruby
   # ActiveSupport 3.x
   "asdf".to_date # => NoMethodError: undefined method `div' for nil:NilClass
   "333".to_date # => NoMethodError: undefined method `div' for nil:NilClass