buffer: reword Buffer.concat error message

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

fixes nodejs#7766.

PR: nodejs#8723
PR-URL: nodejs#8723
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@users.noreply.github.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/buffer.js

@@ -187,7 +187,7 @@ Buffer.isEncoding = function(encoding) {
 
 Buffer.concat = function(list, length) {
   if (!util.isArray(list))
-    throw new TypeError('Usage: Buffer.concat(list[, length])');
+    throw new TypeError('list argument must be an Array of Buffers.');
 
   if (util.isUndefined(length)) {
     length = 0;