Renaming Nuget.org feed key name to Nuget.

fixes https://github.com/aspnet/Universe/issues/174

NuGet.Config

@@ -2,6 +2,6 @@
 <configuration>
   <packageSources>
     <add key="AspNetVNext" value="https://www.myget.org/F/aspnetvnext/api/v2" />
-    <add key="NuGet.org" value="https://nuget.org/api/v2/" />
+    <add key="NuGet" value="https://nuget.org/api/v2/" />
   </packageSources>
-</configuration>
+</configuration>