unison: fix option example

Pairs are named attribute sets. One level was missing.

Fixes #2662

modules/services/unison.nix

@@ -80,10 +80,12 @@ in {
       default = { };
       example = literalExpression ''
         {
+          "my-documents" = {
             roots = [
               "/home/user/documents"
               "ssh://remote/documents"
             ];
+          };
         }
       '';
       description = ''