doc: use correct identifier for callback argument

In one place, the fs docs use `previousStat` for an argument that is
identified as `previous` everywhere else. Use `previous` for
consistency.

PR-URL: #34405
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

doc/api/fs.md

@@ -3996,9 +3996,9 @@ Using [`fs.watch()`][] is more efficient than `fs.watchFile` and
 `fs.unwatchFile` when possible.
 
 When a file being watched by `fs.watchFile()` disappears and reappears,
-then the `previousStat` reported in the second callback event (the file's
-reappearance) will be the same as the `previousStat` of the first callback
-event (its disappearance).
+then the contents of `previous` in the second callback event (the file's
+reappearance) will be the same as the contents of `previous` in the first
+callback event (its disappearance).
 
 This happens when: