fix: Gemma 3 weights conversion vision and multimodal projector paths

[RyanMullins]

What does this PR do?

Fixes bugs in the Gemma 3 weights conversion script:

• Looking for multi-modal projector weights at the wrong location
• Had the wrong prefix on the vision model weights

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@zucchini-nlp

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gemma3

[zucchini-nlp]

Thanks. Not really sure how the directory is organized for conversion so I'll trust in you :)

[HuggingFaceDocBuilderDev]

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