zed: fix typo in variable ZED_POWER_OFF_ENCLO*US*RE_SLOT_ON_FAULT

Replace ENCLO_US_RE with ENCLO_SU_RE in the name of the variable.

Note this changes the user-visible string in zed.rc, thus might
break current users with the wrong string, but it's ~2 months
since zfs-2.2.0 tag is out, thus should not be widespread yet.

Mechanical change:

    $ grep -rl ZED_POWER_OFF_ENCLOUSRE_SLOT_ON_FAULT
    cmd/zed/zed.d/zed.rc
    cmd/zed/zed.d/statechange-slot_off.sh

    $ sed -i 's/ZED_POWER_OFF_ENCLOUSRE_SLOT_ON_FAULT/<linebreak>
                ZED_POWER_OFF_ENCLOSURE_SLOT_ON_FAULT/g' \
      cmd/zed/zed.d/zed.rc \
      cmd/zed/zed.d/statechange-slot_off.sh

    $ grep -rl ZED_POWER_OFF_ENCLOUSRE_SLOT_ON_FAULT
    $

Fixes 11fbcacf37d1a66c7a40bb8920c70ce9a87270ea
("zed: Add zedlet to power off slot when drive is faulted")

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Closes #15651