doc: fix and update examples in http.md

* replace `var` by `const` in http.md
* replace `let` by `const` in http.md
* fix spaces in code examples of http.md
* replace console.log() by .error() in http.md
* make arrow function clearer in http.md
* use object destructuring in http.md
* update output examples in http.md

PR-URL: https://github.com/nodejs/node/pull/12169


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>