My bad, I didn’t edit silkscreen. I don’t think you can do it from within Fritzing, you need to use an svg editer (I use Inkscape) and edit the svg. If you want to learn parts making there are a series of videos, but it has a steep learning curve (depending on where you start). For me with no knowledge of Fritzing or svg editing it was around a year. Mant if the tutorials around are for older versions of Fritzing, these two are for the current version:
meanwhile I’ll add a proper outline to the pcb svg and upload a new version.
Edit:
Easier than I thought. The outline was there, I just failed to resize the svg, so it was outside the view box and thus truncated. If you download the part again you should now have a proper silkscreen outline.