Update intro_prompt_design.ipynb to be compatible with Colab runtime

[dvmorris]

I tried this notebook in Colab, and it is missing the step of setting up the GCP Project ID. This is one technique that could be used, but you might have another preferred method. Let me know what you think!

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[polong-lin]

Thanks for flagging this! We just fixed this across the board for all the notebooks based on your feedback: https://github.com/GoogleCloudPlatform/generative-ai/pull/34