doc: improve randomfill and fix broken link

This patch replaces the word `buf` with `buffer` and fixes the broken
link to `randomfill`.

PR-URL: https://github.com/nodejs/node/pull/12541


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>