Error while using code from docs

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Expected behavior

I have tried code from https://developer.adobe.com/commerce/php/tutorials/frontend/custom-checkout/add-shipping-carrier/#step-3-create-the-carrier-model

While testing with PHP 8.3.4 (cli) (built: Mar 16 2024 08:39:49) (NTS) it is throwing an error

"Fatal Error: 'Type of ---\\Model\\Carrier\\Express::$_code must not be defined (as in class Magento\\Shipping\\Model\\Carrier\\AbstractCarrier)' in '.../Model\/Carrier\/Express.php' on line 18"

"Fatal Error: 'Type of ...\\Model\\Carrier\\Express::$_isFixed must not be defined (as in class Magento\\Shipping\\Model\\Carrier\\AbstractCarrier)' in '.../Model\/Carrier\/Express.php' on line 18"

Should be fixed in docs.

### Actual behavior

It should be corrected

protected string $_code = 'expressdelivery';

protected bool $_isFixed = true;

>>>

protected $_code = 'expressdelivery';

protected $_isFixed = true;

### Steps to reproduce

Create custom module from https://developer.adobe.com/commerce/php/tutorials/frontend/custom-checkout/add-shipping-carrier/#step-3-create-the-carrier-model

Run coomands
Go to cart or checkout

### Environment

- Browser:Chrome
- OS:Linux

### Sample code

protected string $_code = 'expressdelivery';

protected bool $_isFixed = true;

>>>

protected $_code = 'expressdelivery';

protected $_isFixed = true;

### Logs

"Fatal Error: 'Type of ---\\Model\\Carrier\\Express::$_code must not be defined (as in class Magento\\Shipping\\Model\\Carrier\\AbstractCarrier)' in '.../Model\/Carrier\/Express.php' on line 18"

"Fatal Error: 'Type of ...\\Model\\Carrier\\Express::$_isFixed must not be defined (as in class Magento\\Shipping\\Model\\Carrier\\AbstractCarrier)' in '.../Model\/Carrier\/Express.php' on line 18"

Start with the Step 3 “Create the carrier model” section in the linked Adobe Commerce tutorial and compare its sample properties with Magento\Shipping\Model\Carrier\AbstractCarrier. Confirm the PHP 8.3 error and update the documented declarations so the example matches the parent class. Done means the tutorial no longer produces the reported typed-property errors.

Written by the indexing model from the issue text.