fix a typo in Buffer example code

Checklist

• documentation is changed or added
• the commit message follows commit guidelines

Description of change

it shows Buffer(.from[1, 2, 3]); but that doesn't run; should be a typo

1. https://nodejs.org/api/buffer.html#buffer_buffers_and_es6_iteration
2. https://nodejs.org/dist/latest-v5.x/docs/api/buffer.html#buffer_buffers_and_es6_iteration

resolve comment in https://github.com/nodejs/node/commit/c1534e7#commitcomment-17228215; verified the same doc in master 85ab4a5f is correct.