Update cookie struct in 4.0authentication.md
Update cookie struct in 4.0authentication.md

The code of cookie struct is mingled with body text
in 4.0authentication markdown file. Fix that.
thewhitetulip/web-dev-golang-anti-textbook,thewhitetulip/web-dev-golang-anti-textbook,thewhitetulip/web-dev-golang-anti-textbook