i3: Fix escaping in documentation

In nix `${` is escaped like this `''${` and not like this `\${`

modules/services/window-managers/i3-sway/i3.nix

@@ -105,7 +105,7 @@ let
           in lib.mkOptionDefault {
             "''${modifier}+Return" = "exec i3-sensible-terminal";
             "''${modifier}+Shift+q" = "kill";
-            "''${modifier}+d" = "exec \${pkgs.dmenu}/bin/dmenu_run";
+            "''${modifier}+d" = "exec ''${pkgs.dmenu}/bin/dmenu_run";
           }
         '';
       };