Use plot.roc instead of plot.roc_for_far in vulnerability ROC plots

2 jobs for <span class="ref-name">roc-fixes</span> in 16 minutes and 42 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #167920
docker
build_linux_36

00:10:32

passed #167921
macosx
build_macosx_36

00:16:42