doc: Update iostat.md (#498)

iostat的util参数解读有误。参照于man手册 Percentage of elapsed time during which I/O requests were issued to the device (bandwidth utilization for the device). Device saturation occurs when this value is close to 100% for devices serving requests serially.But for devices serving requests in parallel, such as RAID arrays and modern SSDs, this number does not reflect their performance limits.
jaywcjlove · May 22, 2023 · e653d3f · e653d3f
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diff --git a/command/iostat.md b/command/iostat.md
@@ -69,6 +69,6 @@ avgrq-sz | 需求的平均大小区段
 avgqu-sz | 需求的平均队列长度
 await | 等待I/O平均的时间（milliseconds）
 svctm | I/O需求完成的平均时间
-%util | 被I/O需求消耗的CPU百分比
+%util | 设备带宽的使用率，达到100%表示饱和，达到性能瓶颈，如果是支持处理并发请求的设备则不代表性能瓶颈。