update README: extensions menu now present as a standalone option and not in More tools.

<img width="629" alt="Screenshot 2023-08-05 at 11 33 52 PM" src="https://github.com/buildspace/buildspace-os/assets/64839201/6c1dd76d-bd4a-459c-a859-20979ec80893">


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## Version
- Chrome version: 115.0.5790.170
- OS: M1 Mac air (Ventura)



