docs: Add image loading code to multimodal example

[KanishqGandharv219]

Description

Fixes the multimodal example in README by adding missing image loading code.

Problem

The current example uses undefined variables image1 and image2, causing a NameError when users try to run the code.

Solution

• Added from PIL import Image and import numpy as np imports
• Added example code showing how to load images and convert to NumPy arrays
• Matches the format expected by Gemma's ChatSampler (NumPy arrays with uint8 dtype)

Testing

• Verified against official docs: https://gemma-llm.readthedocs.io/en/latest/colab_multimodal.html
• Confirmed NumPy array format is compatible with ChatSampler

Impact

Makes the example copy-pasteable and executable for new users.

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