docs: remove __main__.py reference from readme

[DennisTraub]

Remove outdated reference to src/__main__.py from README

Motivation and Context

The template no longer generates src/__main__.py, but this wasn't reflected in the README. This update ensures documentation matches actual template output.

How Has This Been Tested?

Verified README changes match current template structure.

Breaking Changes

None. Documentation-only change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

None.