docs/zh_CN: remove cn_index tag in mips

It's a unused tag with a incorrect big name but just for mips arch.
So remove it.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210121024113.16344-1-alex.shi@linux.alibaba.com


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