docs: update developer docs to use geminicli GCS bucket (gemini-cli-extensions#101)

Co-authored-by: dishaprakash <57954147+dishaprakash@users.noreply.github.com>

Author: twishabansal

DEVELOPER.md

@@ -33,7 +33,7 @@ The core logic for this extension is handled by a pre-built `toolbox` binary. Th
     VERSION=$(cat toolbox_version.txt)
 
     # Example for macOS/amd64
-    curl -L -o toolbox https://storage.googleapis.com/genai-toolbox/v$VERSION/darwin/amd64/toolbox
+    curl -L -o toolbox https://storage.googleapis.com/genai-toolbox/geminicli/v$VERSION/darwin/amd64/toolbox
     chmod +x toolbox
     ```
     Adjust the URL for your operating system (`linux/amd64`, `darwin/arm64`, `windows/amd64`).