Add magento admin password to credentials example

[BorisovskiP]

Description

As specified in https://developer.adobe.com/commerce/testing/functional-testing-framework/credentials/#add-key-and-value-pair-for-admin-password, magento/MAGENTO_ADMIN_PASSWORD should be added to the .credentials file. So naturally this should also be added to the .credentials.example.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[jilu1]

@magento import pr to magento-commerce/magento2-functional-testing-framework

[m2-github-services]

@jilu1 the Pull Request is successfully imported.