Update README.org w/ new name of claude-agent-acp

[arvidj]

I received the following when following the instructions in the README.

$ npm install -g @zed-industries/claude-agent-acp
npm warn deprecated @zed-industries/claude-agent-acp@0.23.1: This package has been renamed to @agentclientprotocol/claude-agent-acp. Please migrate to continue receiving updates.

This should fix it.

Thank you for contributing to agent-shell!

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

[arvidj]

Also mentioned here: #305 (comment)

[Gleek]

Duplicate #492 😅

[arvidj]

@Gleek Indeed, closing!