doc: fix code examples in zlib.md

* Use test() instead of match() in boolean context.
* Add spaces in code example.
* Fix typo.

PR-URL: https://github.com/nodejs/node/pull/13342


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>