fix genai.GenerativeModel argument name in example

[philippemiron]

Description of the change

Fix wrong argument in __init__.py which shows up on the website.

Motivation

The example on the documentation is not working.

Type of change

Bug fix

Checklist

• [ x ] I have performed a self-review of my code.
• [ x ] I have added detailed comments to my code where applicable.
• [ x ] I have verified that my change does not break existing code.
• [ x ] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [ x ] I am familiar with the Google Style Guide for the language I have coded in.
• [ x ] I have read through the Contributing Guide and signed the Contributor License Agreement.

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[philippemiron]

Added my other email to the CLA.

[MarkDaoust]

Yes, thanks!