Add line break to README migration section

Signed-off-by: Yukai Huang <yukaihuangtw@gmail.com>

Author: Yukaii

README.md

@@ -51,7 +51,7 @@ If you want to contribute to the project, start with [manual deployment](https:/
 CodiMD is highly customizable, learn about all configuration options of networking, security, performance, resources, privilege, privacy, image storage, and authentication in [CodiMD Configuration](https://hackmd.io/c/codimd-documentation/%2Fs%2Fcodimd-configuration).
 
 ### Upgrading and Migration
-Upgrade CodiMD from previous version? See [this guide](https://hackmd.io/c/codimd-documentation/%2Fs%2Fcodimd-upgrade)
+Upgrade CodiMD from previous version? See [this guide](https://hackmd.io/c/codimd-documentation/%2Fs%2Fcodimd-upgrade)<br>
 Migrating from Etherpad? Follow [this guide](https://hackmd.io/c/codimd-documentation/%2Fs%2Fcodimd-migration-etherpad)
 
 ### Developer