jlink option --add-modules should be renamed to --add-root-modules

[Remi Forax]

Hi all,
it's a minor request, but the option --add-modules should be renamed to --add-root-modules because as the doc says it specifies the "Root modules to resolve". Several students of mine had trouble to see the difference between --add-modules and --module-path because of that (and yes they should have read the doc).

regards,
Rémi