Documentation: coding-style: Update syntax highlighting for code-blocks

Use c and elisp instead of none in code-blocks

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240203223926.5077-1-thorsten.blum@toblux.com