fix: update links to https://github.com/google/generative-ai-docs in the readme

[mpursley]

Description of the change

Fix: update links to https://github.com/google/generative-ai-docs in the Readme

Motivation

Type of change

Choose one: Documentation

Checklist

• I have performed a self-review of my code.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.

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