doc: fix examples in buffer.md to avoid confusion

On some systems, some first bytes of allocated buffer can be zeroed
by default, so the example doesn't work well - the buffer looks zeroed
even before the fill.

Fixes: https://github.com/nodejs/node/issues/9786
PR-URL: https://github.com/nodejs/node/pull/9795


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Anna Henningsen <anna@addaleax.net>