Using pidof to determine pid of xray (#136)

from man pidof:
    pidof -- find the process ID of a running program
also, you can use :
    cat /proc/`pidof xray`/limits
in line 163, to find the limits of xray, without the hassle of figuring out the pid yourself.

docs/document/level-2/iptables_gid.md

@@ -163,7 +163,7 @@ _第二条命令：_
 cat /proc/Xray的pid/limits
 ```
 
-找到 max open files 一项，应该是你设置的数值。pid 的获取方法为运行`ps`或`ps -aux`或`ps -a`
+找到 max open files 一项，应该是你设置的数值。pid 的获取方法为运行`ps`或`ps -aux`或`ps -a`或`pidof xray`
 
 服务端和客户端都要检查