doc: favor utf16le over ucs2 in buffer.md

Favor 'utf16le' over its alias 'ucs2' in `buffer.md`.

Ref: https://github.com/nodejs/node/pull/19648#discussion_r178126083

PR-URL: https://github.com/nodejs/node/pull/19688


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>