Update README.md with systemd branch instructions

Author: jakelee8

README.md

@@ -30,10 +30,10 @@ make install
 ## Installation using systemd
 
 ```sh
-wget https://github.com/zfsonlinux/zfs-auto-snapshot/archive/master.zip
-unzip master.zip
-cd zfs-auto-snapshot-master
-make systemd
+wget https://github.com/zfsonlinux/zfs-auto-snapshot/archive/systemd.zip
+unzip systemd.zip
+cd zfs-auto-snapshot-systemd
+make install
 ```
 
 ### Scheduling