OF_node_from_xref.9: Fix function name typo + spdx

MFC after: 3 days
Reported by: Setesh Strong on Community Discord
Fixes: ac32b99c2b9 (Add documentation for OpenFirmware)
Reviewed by: carlavilla, kevans
Approved by: carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D49653