doc/zh_CN: resolve undefined label warning in admin-guide index

Documentation generation warns:

  Documentation/translations/zh_CN/admin-guide/index.rst:3:
  WARNING: undefined label: documentation/admin-guide/index.rst

Use doc reference for .rst files to resolve the warning.

Fixes: 37a607cf ("doc/zh_CN: add admin-guide index")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20200802161956.18268-1-lukas.bulwahn@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/admin-guide/index.rst

 .. include:: ../disclaimer-zh_CN.rst
 
-:Original: :ref:`Documentation/admin-guide/index.rst`
+:Original: :doc:`../../../admin-guide/index`
 :Translator: Alex Shi <alex.shi@linux.alibaba.com>