uses clap attribute missing before clap v3.2

[jonassmedegaard]

Git commit 58d68a8 switched to using clap v3 (nice!) and bumped dependency to version 3.0.
One code line however makes use of syntax only introduced in clap v3.2.

Simply removing the only-recently-supported keyword action - i.e. replacing #[clap(short, long, action)] with #[clap(short, long)] - seems to work fine (read: succesfully compiles).