fix(docs): correct example for GenerateContent using an HTTP URL

[Daquisu]

The current example does not work. It suggests one can directly create a genai.Part from an image URL such as https://storage.googleapis.com/cloud-samples-data/generative-ai/image/scones.jpg, but it is not possible.

One workaround is to download the file and store it in-memory, so we can create a genai.Part from the bytes of a file. This is done in the example for document processing using the same Go SDK: https://ai.google.dev/gemini-api/docs/document-processing#inline_data

I created a repo to show that the current example does not work, and the new example provided by this PR works. See https://github.com/Daquisu/go-genai-example

[google-cla]

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[Daquisu]

I just fixed the CLA issue - sorry about that