buffer: fix typo in lib/buffer.js

PR-URL: https://github.com/nodejs/node/pull/19126


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>