mangohud: fix a non-working example of settingsPerApplication

[KucharczykL]

Description

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [ ] Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• [ ] Test cases updated/added. See example.

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