Rename collector to backend

docs/cn/Quick-start-CN.md

 # 部署步骤
 1. 在Maven Central中下载 `org.apache.skywalking.apm-dist`. 或者本地编译程序，在`dist`目录中找到`skywalking-dist.tar.gz/.zip`.
 1. 部署 Backend
-   1. [单机模式](Deploy-collector-in-standalone-mode-CN.md)
-   1. [集群模式](Deploy-collector-in-cluster-mode-CN.md)
+   1. [单机模式](Deploy-backend-in-standalone-mode-CN.md)
+   1. [集群模式](Deploy-backend-in-cluster-mode-CN.md)
 1. 部署 Java Agent，[doc](Deploy-skywalking-agent-CN.md)
 1. 重启并访问系统功能，查看UI即可。
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docs/en/Quick-start.md

 # Quick start
 1. Download `org.apache.skywalking.apm-dist` in maven central. Or find `skywalking-dist.tar.gz/.zip` in `dist` folder after `mvn clean package`.
 1. Deploy Backend
-   - [Standalone Mode](Deploy-collector-in-standalone-mode.md)
-   - [Cluster Mode](Deploy-collector-in-cluster-mode.md)
+   - [Standalone Mode](Deploy-backend-in-standalone-mode.md)
+   - [Cluster Mode](Deploy-backend-in-cluster-mode.md)
 1. Deploy Java Agent，[doc](Deploy-skywalking-agent.md)
 1. Reboot your applications, and open UI.
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