buffer: fix a typo in Buffer example code

fixes `Buffer(.from[1, 2, 3])` into `Buffer.from(...)` (v5.x only)

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

Commented in:
c1534e7#commitcomment-17228215

PR-URL: #6361
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

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)