periodic/801.trim-zfs: Fix daily-trim-zfs-flags

This variable was named incorrectly, resulting in any specified flags
being silently ignored.

PR: 292074
MFC after: 3 days
Reported by: CrazyMihey@Ya.Ru
Fixes: 493908c4b45c (Add a daily zfs trim script)

(cherry picked from commit 68d6abd9714384a41028dc0d5086b4930366bbea)