Fix example related to `comparison` in Active Record Validations guide

Flipped the example, to show that end_date should be greater than start_date

guides/source/active_record_validations.md

@@ -395,7 +395,7 @@ value, proc, or symbol. Any class that includes Comparable can be compared.
 
 ```ruby
 class Promotion < ApplicationRecord
-  validates :start_date, comparison: { greater_than: :end_date }
+  validates :end_date, comparison: { greater_than: :start_date }
 end
 ```