buffer: reword Buffer.concat error message

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

fixes nodejs#7766.

lib/buffer.js

@@ -181,7 +181,7 @@ 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;