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>

Documentation/drivers/smmstore.md

@@ -22,7 +22,7 @@ The API provides append-only semantics for key/value pairs.
 
 By default SMMSTORE will operate on a separate FMAP region called
 `SMMSTORE`. The default generated FMAP will include such a region.
-On systems with a locked FMAP, e.g. in an existing VBOOT setup
+On systems with a locked FMAP, e.g. in an existing vboot setup
 with a locked RO region, the option exists to add a cbfsfile
 called `smm_store` in the `RW_LEGACY` (if CHROMEOS) or in the
 `COREBOOT` FMAP regions. It is recommended for new builds using