Remove message from ActiveRecord::Rollback example

Do not suggest that raising `ActiveRecord::Rollback` exception should have a message.

There's no point of adding a message to `ActiveRecord::Rollback` exception because it will be rescued by transaction block and the message will be lost.

activerecord/lib/active_record/errors.rb

@@ -326,7 +326,7 @@ class StrictLoadingViolationError < ActiveRecordError
   #           # The system must fail on Friday so that our support department
   #           # won't be out of job. We silently rollback this transaction
   #           # without telling the user.
-  #           raise ActiveRecord::Rollback, "Call tech support!"
+  #           raise ActiveRecord::Rollback
   #         end
   #       end
   #       # ActiveRecord::Rollback is the only exception that won't be passed on