docs/zh_CN: Show the Chinese translation of spinlocks

Last time I forgot to edit locking/index.rst so the Chinese translation
of spinlocks hasn't been shown on web yet.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Yanteng <Si&lt;siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20220629013240.65386-2-yizhou.tang@shopee.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>