Fix incorrect UseCookieAuthentication code snippet

[diddledan]

Replace lambda argument to app.UseCookieAuthentication() with instantiation of a new CookieAuthenticationOptions() object.

[dnfclas]

Hi @diddledan, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, DNFBOT;

[diddledan]

Maybe related to #1472

[dnfclas]

@diddledan, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;

[Rick-Anderson]

Thanks!