buffer: fix a typo in Buffer example code

fixes `Buffer(.from[1, 2, 3])` into `Buffer.from(...)` (v5.x only)

https://nodejs.org/dist/latest-v5.x/docs/api/buffer.html

Commented in:
https://github.com/nodejs/node/commit/c1534e7#commitcomment-17228215

PR-URL: https://github.com/nodejs/node/pull/6361


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>