Adjust comments referencing vm_mem_init()

I cannot find a time where the function was not named this.

Reviewed by: kib, markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45383

(cherry picked from commit deab57178f0b06eab56d7811674176985a8ea98d)