doc: zh_CN: use doc reference to resolve undefined label warning

doc: zh_CN: use doc reference to resolve undefined label warning

Documentation generation warns:

  Documentation/translations/zh_CN/filesystems/debugfs.rst:5:
  WARNING: undefined label: debugfs_index

Use doc reference for files rather than introducing a label to refer to.
This resolves the warning above.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: https://lore.kernel.org/r/20200531183556.5751-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>