docs: remove out-of-dated comment on Config\App::$baseURL

[kenjis]

Description
CI4 doesn't guess the baseURL.

CodeIgniter4/system/HTTP/IncomingRequest.php

Line 485 in ecef7f7

exit('You have an empty or invalid base URL. The baseURL value must be set in Config\App.php, or through the .env file.');

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide