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

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: 6bdbb3b8c6ce ("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.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>