Update chapter_02.rst

修改最大连接数应该是：worker_connections * worker_processes/4

worker_connections
Syntax: worker_connections number

Default:

The worker_connections and worker_processes from the main section allows you to calculate max clients you can handle:

max clients = worker_processes * worker_connections

In a reverse proxy situation, max clients becomes

max clients = worker_processes * worker_connections/4

Since a browser opens 2 connections by default to a server and nginx uses the fds (file descriptors) from the same pool to connect to the upstream backend

参考来源http://wiki.nginx.org/NginxHttpEventsModule#worker_connections