Fixup feeds (#2051)

Co-authored-by: Matt Mitchell <mmitche@microsoft.com>

NuGet.config

@@ -5,7 +5,7 @@
     <add key="dotnet3" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3/nuget/v3/index.json" />
     <add key="dotnet3-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet3-transport/nuget/v3/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources />
 </configuration>