Documentation: Spell vboot all lowercase

Update all occurrences of vboot and spell it lowercase.

Change-Id: I432b0db8a3dda43b71844e557a3d89180f25f1c3
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39799
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/vboot_list/vboot_list.sh

@@ -52,4 +52,4 @@ do
 done
 }
 
-(echo "# VBOOT enabled devices"; generate_vboot_list) > $OUTPUT_FILE
+(echo "# vboot-enabled devices"; generate_vboot_list) > $OUTPUT_FILE