Update README.md

Add a newline to fix the repo file not found error

README.md

@@ -82,7 +82,7 @@ We strongly encourage you to install shadowsocks-libev from `jessie-backports-sl
 For more info about backports, you can refer [Debian Backports](https://backports.debian.org).
 
 ```bash
-sudo sh -c 'printf "deb http://deb.debian.org/debian jessie-backports main" > /etc/apt/sources.list.d/jessie-backports.list'
+sudo sh -c 'printf "deb http://deb.debian.org/debian jessie-backports main\n" > /etc/apt/sources.list.d/jessie-backports.list'
 sudo sh -c 'printf "deb http://deb.debian.org/debian jessie-backports-sloppy main" >> /etc/apt/sources.list.d/jessie-backports.list'
 sudo apt update
 sudo apt -t jessie-backports-sloppy install shadowsocks-libev