ci: T8490: fix typo in grub live-theme (icon_heigh -> icon_height)

`icon_heigh` is a typo of the GRUB2 boot_menu property `icon_height`. Icons are
already disabled in this menu via `icon_width = 0` + `item_icon_space = 0`, so
correcting the property name is behaviour-neutral. Clears the lone typos-check
hit on this repo ahead of the T8490 ruleset pilot active flip.

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Author: andamasov

data/live-build-config/bootloaders/grub-pc/live-theme/theme.txt

@@ -31,7 +31,7 @@ terminal-font: "Unifont Regular 16"
         item_padding = 0
         item_spacing = 4
 	icon_width = 0
-	icon_heigh = 0
+	icon_height = 0
 	item_icon_space = 0
 }