docs: deprecated.rst: Update an example

vmalloc() has a 2-factor form. It is vmalloc_array().
So use another function as an example.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/3484e46180dd2cf05d993ff1a78b481bc2ad1f71.1687723931.git.christophe.jaillet@wanadoo.fr