docs/zh_CN: fix one sentence in llvm.rst

According to Jinjie Ruan, the description about the prebuilt llvm is
incorrect. Fix this sentence with new understanding.

Link: https://groups.google.com/g/hust-os-kernel-patches/c/GbN5RkVrBho/m/2sC9rAw7BQAJ


Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241107013615.374757-1-dzm91@hust.edu.cn