Fix inforef command
The inforef command is not using the proper syntax which fails the build. Fix suggested by Aidan Kehoe in IRC.
This MR just does the minimal change to make it build properly. inforef command as such is deprecated and other constructs should be used for a proper fix. See https://www.gnu.org/savannah-checkouts/gnu/texinfo/manual/texinfo/html_node/_0040inforef.html