Fix outdated dockerfile reference

Author: tcmal

scripts/generate.sh

@@ -1,3 +1,3 @@
 #!/usr/bin/env sh
 cd "$(dirname "$0")/.."
-docker run -it --rm -v $(pwd):/material:ro -v $(pwd)/out:/out ghcr.io/edinburghhacklab/hacklab-training:master /material /out $@
+docker run -it --rm -v $(pwd):/material:ro -v $(pwd)/out:/out ghcr.io/edinburghhacklab/hacklab-training:main /material /out $@