hardcoded mirror is deb.debian.org, use hetzner here

howto/new-machine-hetzner-robot.md

@@ -53,7 +53,8 @@ repository. Here's an example of a commandline:
               --fqdn=HOSTNAME.torproject.org \
               --fai-disk-config=installer/disk-config/gnt-fsn-NVMe \
               --package-list=installer/packages \
-              --post-scripts-dir=installer/post-scripts/
+              --post-scripts-dir=installer/post-scripts/ \
+              --mirror=https://mirror.hetzner.de/debian/packages/
 
 Taking that apart: