Documentation/ABI: sysfs-class-firmware-attributes: Fix Sphinx errors

Fix the following warnings from "make htmldocs":

Documentation/ABI/testing/sysfs-class-firmware-attributes:130:
  ERROR: Unexpected indentation.
  ERROR: Unexpected indentation.
  ERROR: Unexpected indentation.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20220324164737.21765-3-hdegoede@redhat.com