doc: fix stylistic issues in api/net.md

* Change var to const in an example of server creation.
* Add missing semicolons.
* Use `console` syntax highlighting in `telnet` and `nc` invocation
  examples and add shell prompt symbols to be consistent with the rest
  of the documentation.

PR-URL: https://github.com/nodejs/node/pull/11786


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>