[doc] Move class definition to the correct section

doc/providers/validator.rst

@@ -78,18 +78,6 @@ collection of constraints::
 
     use Symfony\Component\Validator\Constraints as Assert;
 
-    class Book
-    {
-        public $title;
-        public $author;
-    }
-
-    class Author
-    {
-        public $first_name;
-        public $last_name;
-    }
-
     $book = array(
         'title' => 'My Book',
         'author' => array(
@@ -123,6 +111,18 @@ the class properties and getters, and then call the ``validate`` method::
 
     use Symfony\Component\Validator\Constraints as Assert;
 
+    class Book
+    {
+        public $title;
+        public $author;
+    }
+
+    class Author
+    {
+        public $first_name;
+        public $last_name;
+    }
+
     $author = new Author();
     $author->first_name = 'Fabien';
     $author->last_name = 'Potencier';