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: nodejs#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>

doc/api/deprecations.md

@@ -152,7 +152,7 @@ Type: Runtime
 explicitly via error event handlers set on the domain instead.
 
 <a id="DEP0013"></a>
-### DEP0013: fs async function without callback
+### DEP0013: fs asynchronous function without callback
 
 Type: Runtime