zfsboot: fix misuse of the phrase zpool

A pool is not a zpool.

Reviewed by: jbh, ivy
Approved by: des (mentor)
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1569

(cherry picked from commit f69afe4201ff33da0b3c3ca464a92e2e0b07ac43)