docs: improve Upgrade Models sample code

[kenjis]

Description

• The current CI4 sample code https://codeigniter.com/user_guide/installation/upgrade_models.html#code-example does not work because the insert() method parameters are different.
• public function insert($name, $address, $email) cannot be defined in CI4 model because it is already defined.

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

[datamweb]

LGTM!