docs: update sample code for validation placeholder

Author: kenjis

user_guide_src/source/models/model/038.php

@@ -7,6 +7,7 @@
 class MyModel extends Model
 {
     protected $validationRules = [
-        'email' => 'required|valid_email|is_unique[users.email,id,{id}]',
+        'id'    => 'max_length[19]|is_natural_no_zero',
+        'email' => 'required|max_length[254]|valid_email|is_unique[users.email,id,{id}]',
     ];
 }

user_guide_src/source/models/model/040.php

@@ -7,6 +7,7 @@
 class MyModel extends Model
 {
     protected $validationRules = [
-        'email' => 'required|valid_email|is_unique[users.email,id,4]',
+        'id'    => 'max_length[19]|is_natural_no_zero',
+        'email' => 'required|max_length[254]|valid_email|is_unique[users.email,id,4]',
     ];
 }