Update README.md (#12684)

Fix casing in agents core pkg

### Motivation and Context

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### Description

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### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Co-authored-by: SergeyMenshykh <[email protected]>

Author: AndreasVolkmann