Fix example nuget.config (PowerShell#14349)

Author: rjmholt

docs/host-powershell/sample/NuGet.config

@@ -1,8 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" /> 
-    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
-    <add key="powershell-core" value="https://powershell.myget.org/F/powershell-core/api/v3/index.json" />
   </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>