signal-desktop: fix app icon (#310587)

Co-authored-by: éclairevoyant <848000+eclairevoyant@users.noreply.github.com>

pkgs/applications/networking/instant-messengers/signal-desktop/generic.nix

@@ -158,7 +158,8 @@ stdenv.mkDerivation rec {
 
     # Fix the desktop link
     substituteInPlace $out/share/applications/${pname}.desktop \
-      --replace "/opt/${dir}/${pname}" $out/bin/${pname}
+      --replace "/opt/${dir}/${pname}" $out/bin/${pname} \
+      --replace-fail "StartupWMClass=Signal" "StartupWMClass=signal"
 
     # Note: The following path contains bundled libraries:
     # $out/lib/${dir}/resources/app.asar.unpacked/node_modules/