doc: clarify buffer class

Fixes: https://github.com/nodejs/node/issues/6891
PR-URL: https://github.com/nodejs/node/pull/6914
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/buffer.md

@@ -18,7 +18,7 @@ The size of the `Buffer` is established when it is created and cannot be
 resized.
 
 The `Buffer` class is a global within Node.js, making it unlikely that one
-would need to ever use `require('buffer')`.
+would need to ever use `require('buffer').Buffer`.
 
 ```js
 const buf1 = Buffer.alloc(10);