modules/helix: fix pkill line on Darwin

On Darwin the previous ordering resulted in `-U` being parsed as a flag
with the value `SR1`. Moving the signal flag to the start of the command
fixes this issue, and pkill from procps doesn't care.

modules/programs/helix.nix

@@ -234,7 +234,7 @@ in
     xdg.configFile =
       let
         pkillPrefix = if pkgs.stdenv.hostPlatform.isDarwin then "/usr" else pkgs.procps;
-        onChange = "${pkillPrefix}/bin/pkill -u $USER -x -USR1 '(hx|\\.hx-wrapped)' || true";
+        onChange = "${pkillPrefix}/bin/pkill -USR1 -u $USER -x '(hx|\\.hx-wrapped)' || true";
 
         settings =
           let