doc: fix a formatting error in buffer.md

It was treated as a link to "e.g. `Uint8Array.from()`".

PR-URL: https://github.com/nodejs/node/pull/8553


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>