docs: fix Composio register_tools agent order

[LiJzd]

Fixes #5572

Summary

• Updates the Composio email-agent notebook on the 0.2 branch.
• Registers the assistant as the caller and the UserProxyAgent as the executor.
• Adjusts the surrounding explanation to match the corrected agent roles.

Duplicate-work check

• Checked microsoft/autogen#5572.
• Checked open PRs with 5572 OR Composio caller executor: no matches.
• Checked open PRs with Composio notebook example: no matches.

Validation

• python -m json.tool notebook\agentchat_function_call_with_composio.ipynb
• git diff --check
• Local text check confirmed caller=chatbot and executor=user_proxy.

I did not execute the notebook locally because it requires external Composio and Gmail credentials.

AI assistance

I used OpenAI Codex to inspect the issue, check for duplicate PRs, make the notebook edit, and run the validation above. I reviewed the final diff before submitting.