updated the pakcage source for rc2 release

NuGet.config

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
-    <add key="AspNetVNext" value="https://www.myget.org/F/aspnetcidev/api/v3/index.json" />
-    <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
+    <add key="aspnetcore" value="https://www.myget.org/F/aspnetrc2/api/v3/index.json" />
   </packageSources>
-</configuration>
+</configuration>