Fix comment (#1436)

Fix comment, it's setting sign in scheme (old left-over from copy-paste?)

src/Microsoft.AspNetCore.Authentication/AuthenticationServiceCollectionExtensions.cs

@@ -85,7 +85,7 @@ namespace Microsoft.Extensions.DependencyInjection
             return services.AddScheme<TOptions, THandler>(authenticationScheme, displayName, configureScheme: null, configureOptions: configureOptions);
         }
 
-        // Used to ensure that there's always a default data protection provider
+        // Used to ensure that there's always a sign in scheme
         private class EnsureSignInScheme<TOptions> : IPostConfigureOptions<TOptions> where TOptions : RemoteAuthenticationOptions
         {
             private readonly AuthenticationOptions _authOptions;