nixos/meta: Fix maintainers example

`maintainers.all` does not exist, and should not exist.

Neither does `alice`, but she's a metavariable.

nixos/modules/misc/meta.nix

···
         type = listOfMaintainers;
         internal = true;
         default = [ ];
-        example = lib.literalExpression ''[ lib.maintainers.all ]'';
+        example = lib.literalExpression ''[ lib.maintainers.alice ]'';
         description = ''
           List of maintainers of each module.  This option should be defined at
           most once per module.