ppp: Fix spelling typo in comment in ppp_async_encode()

Fix spelling typo in comment

Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: liyouhong <liyouhong@kylinos.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231227015831.289077-1-liyouhong@kylinos.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>