buffer: reword Buffer.concat error message

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

fixes joyent/node#7766.

PR: #8723
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>

lib/buffer.js

@@ -475,7 +475,7 @@ Buffer.prototype.fill = function fill(value, start, end) {
 
 Buffer.concat = function(list, length) {
   if (!Array.isArray(list)) {
-    throw new TypeError('Usage: Buffer.concat(list, [length])');
+    throw new TypeError('list argument must be an Array of Buffers.');
   }
 
   if (list.length === 0) {