Fix DirectML package name in genai/howto/install.md (#20771)

### Description
Fix package name 
from ```dotnet add package Microsoft.ML.OnnxRuntimeGenAI.*Cuda*
--prerelease``` to ```dotnet add package
Microsoft.ML.OnnxRuntimeGenAI.DirectML --prerelease``` in DirectML
section.

### Motivation and Context
Clearly confusing for beginners if not fixed (will break further processes)

docs/genai/howto/install.md

@@ -48,7 +48,7 @@ dotnet add package Microsoft.ML.OnnxRuntimeGenAI.Cuda --prerelease
 For the package that has been optimized for DirectML:
 
 ```bash
-dotnet add package Microsoft.ML.OnnxRuntimeGenAI.Cuda --prerelease
+dotnet add package Microsoft.ML.OnnxRuntimeGenAI.DirectML --prerelease
 ```