docs/zh_CN: Fix '.. only::' directive's expression

docs/zh_CN: Fix '.. only::' directive's expression

Commit febe6c2f859e ("docs/zh_CN: Add translation
zh_CN/doc-guide/index.rst") translated ".. only::" directive too much.
Use the one as found in the original doc-guide/index.rst.

Fixes: febe6c2f859e ("docs/zh_CN: Add translation zh_CN/doc-guide/index.rst")
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Wu XiangCheng <bobwxc@email.cn>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Alex Shi <alexs@kernel.org>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/20221205032622.8697-1-akiyks@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>