chore: fix link in readme

Author: averikitsch

README.md

@@ -5,7 +5,7 @@
 
 Developers can effortlessly connect, interact, and generate data insights with [BigQuery](https://cloud.google.com/bigquery/docs) datasets and data using natural language commands.
 
-Learn more about [Gemini CLI Extensions](https://github.com/google-gemini/gemini-cli/blob/main/docs/extension.md).
+Learn more about [Gemini CLI Extensions](https://github.com/google-gemini/gemini-cli/blob/main/docs/extensions/index.md).
 
 ## Why Use the BigQuery Data Analytics Extension?