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<siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20220629013240.65386-2-yizhou.tang@shopee.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/locking/index.rst

@@ -14,6 +14,8 @@
 .. toctree::
     :maxdepth: 1
 
+    spinlocks
+
 TODOList:
 
     * locktypes
@@ -24,7 +26,6 @@ TODOList:
     * rt-mutex-design
     * rt-mutex
     * seqlock
-    * spinlocks
     * ww-mutex-design
     * preempt-locking
     * pi-futex