Documentation : Update relay function types

This patch updates two callback functions provided as an example in
relay API documentation : subbuf_start and create_buf_file_handler.

These functions were using older and incorrect types causing an
"initialization from incompatible pointer type".

Signed-off-by: Yohan Pipereau <yohan.pipereau@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>