Fixed typo in README.md (#12735)

### Description

Fixed the casing of word Core in the name of the package.

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] 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
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Author: abatishchev

README.md

@@ -55,7 +55,7 @@ pip install semantic-kernel
 
 ```bash
 dotnet add package Microsoft.SemanticKernel
-dotnet add package Microsoft.SemanticKernel.Agents.core
+dotnet add package Microsoft.SemanticKernel.Agents.Core
 ```
 
 ### Java