doc: fix buffer[index] example

PR-URL: #5253
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/buffer.markdown

@@ -351,7 +351,7 @@ for (var i = 0; i < str.length ; i++) {
   buf[i] = str.charCodeAt(i);
 }
 
-console.log(buf);
+console.log(buf.toString('ascii'));
   // Prints: Node.js
 ```