minor: fix RazorWebSite example to work on .NET Core 3
minor: fix RazorWebSite example to work on .NET Core 3

andrewlock/NetEscapades.AspNetCore.SecurityHeaders,andrewlock/NetEscapades.AspNetCore.SecurityHeaders,andrewlock/NetEscapades.AspNetCore.SecurityHeaders,andrewlock/NetEscapades.AspNetCore.SecurityHeaders