[9.18] fix: usr: RPZ canonical warning displays zone entry incorrectly

When an IPv6 rpz prefix entry is entered incorrectly the log
message was just displaying the prefix rather than the full
entry.  This has been corrected.

Closes #5491

Backport of MR !10890

Merge branch 'backport-5491-rpz-canonical-warning-displays-zone-entry-incorrectly-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10931