buffer: fix a typo in Buffer example code

it shows `Buffer(.from[1, 2, 3]);` but that doesn't run; should be a typo

https://nodejs.org/dist/latest-v5.x/docs/api/buffer.html#buffer_buffers_and_es6_iteration

resolve comment in c1534e7#commitcomment-17228215 the same doc in master 85ab4a5 is correct.

doc/api/buffer.md

@@ -222,7 +222,7 @@ function:
 Buffers can be iterated over using the ECMAScript 2015 (ES6) `for..of` syntax:
 
 ```js
-const buf = Buffer(.from[1, 2, 3]);
+const buf = Buffer.from([1, 2, 3]);
 
 for (var b of buf)
   console.log(b)