doc: improve text in fs docs about omitting callbacks

Repair misplaced colon and condense text.

PR-URL: https://github.com/nodejs/node/pull/34307


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>