doc: use `foo.prototype.bar` notation in buffer.md

Most of the documentation uses `foo.prototype.bar` notation instead of
`foo#bar` notation, this commit apply the former in `buffer.md`.

PR-URL: https://github.com/nodejs/node/pull/38032


Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>