Renaming Nuget.org feed key name to Nuget.
Renaming Nuget.org feed key name to Nuget.

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

aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore