doc: clarify buffer class

Fixes: #6891
PR-URL: #6914
Reviewed-By: Claudio Rodriguez <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Сковорода Никита Андреевич <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>

Author: stevemao

doc/api/buffer.markdown

@@ -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 = new Buffer(10);