riscv/vdso: typo therefor

The adverbs 'therefor' and 'therefore' have different meaning.
As the meaning here is 'consequently' the spelling should be 'therefore'.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220925004757.9089-1-heinrich.schuchardt@canonical.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>