doc: extensions: zephyr-domain: fix object descriptions

Zephyr's domain code-sample object description was incorrectly yielded,
making Sphinx inventory (objects.inv) unusable on other projects that
need to use the domain via Intersphinx.

Ref. https://www.sphinx-doc.org/en/master/extdev/domainapi.html



Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>