nixos/jigasi: update to use literalExpression

I saw this trace when building my system configuration this morning:

```
lazy-options.json> trace: warning: literalExample is deprecated, use literalExpression instead, or use literalMD for a non-Nix description.
```

This warning was introduced in https://github.com/NixOS/nixpkgs/pull/237557.

The option this commit changes was introduced in https://github.com/NixOS/nixpkgs/pull/137003.

nixos/modules/services/networking/jigasi.nix

@@ -96,7 +96,7 @@ in
     config = mkOption {
       type = attrsOf str;
       default = { };
-      example = literalExample ''
+      example = literalExpression ''
         {
           "org.jitsi.jigasi.auth.URL" = "XMPP:jitsi-meet.example.com";
         }