doc/zh_CN: fix title heading markup in admin-guide cpu-load

Documentation generation warns:

  Documentation/translations/zh_CN/admin-guide/cpu-load.rst:1:
  WARNING: Title overline too short.

Extend title heading markup by one. It was just off by one.

Fixes: e210c66d ("doc/zh_CN: add cpu-load Chinese version")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Tao Zhou <ouwen210@hotmail.com>
Link: https://lore.kernel.org/r/20200802162101.18875-1-lukas.bulwahn@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/admin-guide/cpu-load.rst

-=======
+========
 CPU 负载
-=======
+========
 
 Linux通过``/proc/stat``和``/proc/uptime``导出各种信息，用户空间工具
 如top(1)使用这些信息计算系统花费在某个特定状态的平均时间。