mcomix: fix invalid .desktop icon name
mcomix: fix invalid .desktop icon name

The postInstall step for this package copies an icon to the hicolor icon
theme under the name mcomix3.png (${pname}). The applications
mcomix.desktop references the icon by Icon=mcomix. The means the copied
icon ends up with the "wrong" filename making lookup following the xdg
standard break. This change updates the icon reference in the .desktop
file to match what is currently being installed.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs