doc: zh_CN: add tmpfs to index tree

Add tmpfs to the index tree while adding tmppfs translation.
Signed-off-by: Wang Qing <wangqing@vivo.com>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: https://lore.kernel.org/r/1605509272-13770-1-git-send-email-wangqing@vivo.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/filesystems/index.rst

@@ -25,4 +25,5 @@ Linux Kernel中的文件系统
 
    virtiofs
    debugfs
+   tmpfs