doc: add missing changes: entry for mkdir

Refs: https://github.com/nodejs/node/pull/31530

PR-URL: https://github.com/nodejs/node/pull/32490


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>