docs: fix mcp sever command in readme

[averikitsch]

Description

Should include a concise description of the changes (bug or feature), it's
impact, along with a summary of the solution

PR Checklist

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🛠️ Fixes #<issue_number_goes_here>

[gemini-code-assist]

Code Review

This pull request updates the README.md by adding the "--stdio" flag to the arguments of the @toolbox-sdk/server configuration example. I have no feedback to provide on the code changes.

[github-actions]

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