docs(vertex): pass model to from_genai

[coyaSONG]

Describe your changes

Correct the Vertex AI migration example so model is passed to Instructor's from_genai helper instead of google.genai.Client.

This matches the two APIs: the Google client configures the Vertex AI connection, while Instructor uses model as the default model for requests.

Closes #2416.

Issue ticket number and link

• [Docs]: Incorrect documentation for google.genai with vertex #2416: [Docs]: Incorrect documentation for google.genai with vertex #2416

Verification

• Parsed the updated Python code block with ast.parse
• Confirmed the google-genai v1.31.0 Client signature does not accept model
• Confirmed Instructor's from_genai helper accepts model
• Ran git diff --check

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests. (Not applicable; documentation-only change.)
• If it is a core feature, I have added documentation.

This PR was prepared with OpenAI Codex.