Update missing documentation

Author: ernoaapa

docs/installation.md

@@ -17,7 +17,7 @@ By far the easiest and most secure way to use Eliot is by using [EliotOS](elioto
 
 1. Format sdcard as you would normally
 2. Mount it to for example `/Volumes/rpi3`
-3. Build image and unpack it to the directory `eli build device | tar xv -C /Volumes/rpi3`
+3. Build image and unpack it to the directory `eli build device --format tar | tar xv -C /Volumes/rpi3`
 4. Unmount the disk
 5. Connect RaspberryPI with ethernet cable to same network with your laptop and power on!
 6. In less than 10s you should see the device with command `eli get devices`