docs: warn node fix
docs: warn node fix

* Fixes the warn_node signature error in sphinx.

Signed-off-by: Krzysztof Nowak <ec2e6b092344a8f9a548dde0d2ede1e06ecfeabd@cern.ch>

tiborsimko/invenio-records,inveniosoftware/invenio-records,inveniosoftware/invenio-records,tiborsimko/invenio-records