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>

lib/buffer.js

@@ -170,7 +170,7 @@ Buffer.isEncoding = function(encoding) {
 
 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 (length === undefined) {
     length = 0;