can-doc: Fix wrong chapter reference

In f35f6c8f

 (can: update MAINTAINERS and Documentation) chapter 3.3
was removed. This patch fixes some old references to chapter 3.4 which
no longer exists.

Signed-off-by: Stefan Tatschner <stefan@sevenbyte.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>