puc: fix man page reference

The reference to puc(9) is wrong; the page does not exist, and drivers
belong to section 4. Change the reference to puc(4), which does exist.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 09e5d91069d13000d1d93b0e6a26731926d500e1)