Fix example for xkbOptions

###### Motivation for this change
With space between two options, multiple options just don't work
Looks like xkbOptions then used for generation of xorg.conf.d/00-keyboard.conf.
kbd's man says XkbOptions must be comma-separated without spaces.
https://linux.die.net/man/4/kbd

###### Things done

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###### Notify maintainers

cc @

nixos/modules/services/x11/xserver.nix

@@ -333,7 +333,7 @@ in
       xkbOptions = mkOption {
         type = types.str;
         default = "terminate:ctrl_alt_bksp";
-        example = "grp:caps_toggle, grp_led:scroll";
+        example = "grp:caps_toggle,grp_led:scroll";
         description = ''
           X keyboard options; layout switching goes here.
         '';