Bump coverlet.collector from 6.0.2 to 6.0.3 in /docs/core/tutorials/snippets/library-with-visual-studio/csharp/StringLibraryTest in the dotnet group

Bump coverlet.collector from 6.0.2 to 6.0.3 in /docs/core/tutorials/snippets/library-with-visual-studio/csharp/StringLibraryTest in the dotnet group #10133