update document

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

 import com.xxl.job.core.thread.JobThread;
 import com.xxl.job.core.thread.TriggerCallbackThread;
 import com.xxl.rpc.registry.ServiceRegistry;
-import com.xxl.rpc.remoting.invoker.XxlRpcInvokerFactory;
 import com.xxl.rpc.remoting.net.impl.netty_http.server.NettyHttpServer;
 import com.xxl.rpc.remoting.provider.XxlRpcProviderFactory;
 import com.xxl.rpc.serialize.Serializer;