pmap_extract.9: Fix pmap_extract_and_hold()'s function type

pmap_extract_and_hold() returns a vm_page_t instead of a physical page
address.

Sponsored by: The FreeBSD Foundation
Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D31691

(cherry picked from commit 6e1df1d14c6dfcc209c1416ec0832e4d08191c72)