buffer: reword Buffer.concat error message

this brings the error messaging in line with
other node TypeError messages.

Fixes: https://github.com/joyent/node/issues/7766
PR-URL: https://github.com/joyent/node/pull/8723


Reviewed-By: James M Snell <jasnell@users.noreply.github.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>