Change Debian ISO url to be generated from the contents of SHA512SUM.

This removes the need to maintain the version number, which is currently
out of date.

README.md

@@ -101,11 +101,11 @@ This guide recommends using a bootable "live" Debian Linux image to provide such
 To use Debian Live, download the latest image:
 
 ```console
-$ curl -LfO https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/debian-live-10.4.0-amd64-xfce.iso
-
 $ curl -LfO https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/SHA512SUMS
 
 $ curl -LfO https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/SHA512SUMS.sign
+
+$ curl -LfO https://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid/$(awk '/xfce.iso/ {print $2}' SHA512SUMS)
 ```
 
 Verify the signature of the hashes file with GPG: