update document

xxl-job-core/src/main/java/com/xxl/job/core/executor/XxlJobExecutor.java

 
         xxlRpcProviderFactory.setServer(NettyHttpServer.class);
         xxlRpcProviderFactory.setSerializer(HessianSerializer.class);
-        xxlRpcProviderFactory.setCorePoolSize(30);
+        xxlRpcProviderFactory.setCorePoolSize(20);
         xxlRpcProviderFactory.setMaxPoolSize(200);
         xxlRpcProviderFactory.setIp(ip);
         xxlRpcProviderFactory.setPort(port);