docs/zh_CN: fix a broken reference

Warning: Documentation/translations/zh_CN/kbuild/index.rst
references a file that doesn't exist: Documentation/kbuild/index

Fix this by adding its full name.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407300812.1VvDFdxD-lkp@intel.com/Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240730053652.4073433-1-dzm91@hust.edu.cn

Documentation/translations/zh_CN/kbuild/index.rst

@@ -2,7 +2,7 @@
 
 .. include:: ../disclaimer-zh_CN.rst
 
-:Original: Documentation/kbuild/index
+:Original: Documentation/kbuild/index.rst
 :Translator: 慕冬亮 Dongliang Mu <dzm91@hust.edu.cn>
 
 ============