Fix README path reference from Demos to Samples

[Copilot]

Motivation and Context

Addresses review feedback from PR #4064 (comment #4064 (comment)) regarding an outdated path reference that was missed during the Demos → Samples migration.

Description

Updated dotnet/src/Shared/Samples/README.md line 20 to reference the correct Samples folder path instead of the obsolete Demos folder:

- <Compile Include="$(MSBuildThisFileDirectory)\..\src\Shared\Demos\*.cs" LinkBase="" Visible="false" />
+ <Compile Include="$(MSBuildThisFileDirectory)\..\src\Shared\Samples\*.cs" LinkBase="" Visible="false" />

This ensures the documentation accurately reflects the current repository structure for users copying SampleEnvironment.cs to external projects.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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