riscv: vector: Fix a typo of preempt_v

The term "preempt_v" represents the RISCV_PREEMPT_V field of riscv_v_flags
and is used in lots of comments.

Here corrects the miss-spelling "prempt_v". And s/acheived/achieved/.

Reviewed-by: Andy Chiu <andybnac@gmail.com>
Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Link: https://lore.kernel.org/r/20240221100252.3990445-1-songshuaishuai@tinylab.org


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