tdesktop: fix icon path

[cruizh]

Motivation for this change

The .desktop file now refers to the icon as telegram instead of telegram-desktop. This caused a placeholder icon to be shown instead.

Things done

Changed path of the tdesktop icon.

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[primeos]

Thanks :)

For some reason it was still working with gnome3 and palasma5 (at least for me) but this change definitely makes sense.