“Risks of proprietary software”: use `—` instead of `--`

This improves !273 (merged) by using em-dashes as per https://en.wikipedia.org/wiki/Dash#Simple_equivalence_(or_near-equivalence)_of_colon_and_em_dash

I have verified that this was the only occurrence of -- in Discover's .qml files.

Full disclosure: I'm not a native English speaker. 😄