Update GenerativeModel.md

[ShotaHirabayashi]

we don't have to put this import. maybe Unnecessary imports confuse beginners

Description of the change

remove import PIL.Image. maybe Unnecessary imports confuse beginners.

Motivation

to reduce confusion beginners.

Type of change

Documentation

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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