docs/zh_CN: add completion translation

Translate .../scheduler/completion.rst into Chinese.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Acked-by: Tang Yizhou <tangyizhou@huawei.com>
Link: https://lore.kernel.org/r/47b6fd1f277058dfb8d06dd3d33fedc2ef55fcbb.1638777809.git.siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/scheduler/index.rst

@@ -17,10 +17,11 @@ Linux调度器
 .. toctree::
     :maxdepth: 1
 
+    completion
+
 
 TODOList:
 
-    completion
     sched-arch
     sched-bwc
     sched-deadline