"bellow" -> "below"

The recommendation from kegel.com/kerspell is to only fix the howlers.
"Bellow" is a synonym of "howl" so this should be appropriate.
Signed-off-by: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>