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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- [ ] Tested using sandboxing ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS, or option `sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file) on non-NixOS linux)
- Built on platform(s)
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   - [ ] macOS
   - [ ] other Linux distributions
- [ ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
- [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nixpkgs-review --run "nixpkgs-review wip"`
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- [ ] Determined the impact on package closure size (by running `nix path-info -S` before and after)
- [ ] Ensured that relevant documentation is up to date
- [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).

###### Notify maintainers

cc @

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/modules/services/x11/xserver.nix b/nixos/modules/services/x11/xserver.nix
index 1f6ee7cfffd..730a7431321 100644
--- a/nixos/modules/services/x11/xserver.nix
+++ b/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.
         '';