doc: fix typo in buffer.md

PR-URL: https://github.com/nodejs/node/pull/37268


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>