Added missing nuget.org default feed

Author: Sergio0694

samples/NuGet.config

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
     <packageSources>
+        <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
         <add key="WindowsCommunityToolkit-CI" value="https://pkgs.dev.azure.com/dotnet/WindowsCommunityToolkit/_packaging/WindowsCommunityToolkit-MainLatest/nuget/v3/index.json" />
     </packageSources>
 </configuration>