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: https://github.com/nodejs/node/pull/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>