[site] fix link pointing to google_generative_ai package

[cem256]

trafficstars

The link leads to a 404 error

Description of what this PR is changing or adding, and why:

While reading the documentation, I found following link returned a 404 error https://pub.dev/packages/packages/google_generative_ai

This PR attempts to fix the broken link for the google_generative_ai package.

Presubmit checklist

• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.

[google-cla[bot]]

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[parlough]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit c2535d4ad8352dc17636e7bac2e029befa8765a9):

https://flutter-docs-prod--pr10623-patch-1-cf1vn1r1.web.app