doc: clarify async/asynchronous in deprecations.md

Change _async_ to _asynchronous_ in `deprecations.md` to avoid confusion
with the `async` keyword. This has the additional minor benefit of being
more formal and less conversational prose.

PR-URL: https://github.com/nodejs/node/pull/15172


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>