Reordered classes in AR Validation #validates_with example [ci skip]

Person called GoodnessValidator before it was defined.
This change will compile the example correctly.

guides/source/active_record_validations.md

@@ -616,10 +616,6 @@ The default error message is _"has already been taken"_.
 This helper passes the record to a separate class for validation.
 
 ```ruby
-class Person < ActiveRecord::Base
-  validates_with GoodnessValidator
-end
-
 class GoodnessValidator < ActiveModel::Validator
   def validate(record)
     if record.first_name == "Evil"
@@ -627,6 +623,10 @@ class GoodnessValidator < ActiveModel::Validator
     end
   end
 end
+
+class Person < ActiveRecord::Base
+  validates_with GoodnessValidator
+end
 ```
 
 NOTE: Errors added to `record.errors[:base]` relate to the state of the record
@@ -644,10 +644,6 @@ Like all other validations, `validates_with` takes the `:if`, `:unless` and
 validator class as `options`:
 
 ```ruby
-class Person < ActiveRecord::Base
-  validates_with GoodnessValidator, fields: [:first_name, :last_name]
-end
-
 class GoodnessValidator < ActiveModel::Validator
   def validate(record)
     if options[:fields].any?{|field| record.send(field) == "Evil" }
@@ -655,6 +651,10 @@ class GoodnessValidator < ActiveModel::Validator
     end
   end
 end
+
+class Person < ActiveRecord::Base
+  validates_with GoodnessValidator, fields: [:first_name, :last_name]
+end
 ```
 
 Note that the validator will be initialized *only once* for the whole application