Bad path for TestShop host in docs/contributing.md

The `docs/contributing.md` document shows the following command to run the TestShop host program:

```shell
dotnet restore playground/TestShop/AppHost/AppHost.csproj
dotnet run --project playground/TestShop/AppHost/AppHost.csproj
```

These commands are incorrect as the correct path is `playground/TestShop/TestShop.AppHost/TestShop.AppHost.csproj`.