doc: fix buf.length slice example

Previously tried to reassign a const.

PR-URL: #5259
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/buffer.markdown

@@ -604,7 +604,7 @@ modify the length of a Buffer should therefore treat `length` as read-only and
 use [`buf.slice()`][] to create a new Buffer.
 
 ```js
-const buf = new Buffer(10);
+var buf = new Buffer(10);
 buf.write('abcdefghj', 0, 'ascii');
 console.log(buf.length);
   // Prints: 10