docs/zh_CN: fix an inconsistent statement in dev-tools/testing-overview

This patch fixes an inconsistent describtion in testing-overview.rst,
which should be ``kmalloc`` instead of ``kmalloc_arry`` according to
the original text.

Signed-off-by: LIU Haoyang <tttturtleruss@gmail.com>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: Alex Shi <alexs@kernel.org>