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/releases/coreboot-4.8.1-relnotes.md

@@ -77,7 +77,7 @@ Security
 --------
 * Start of refactoring the TPM software stack
 * Introduced coreboot security section in kconfig
-* VBoot & TPM code moved into src/security
+* vboot & TPM code moved into src/security
 
 Intelmetool
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