stylix: only read autoEnable in mkEnableWallpaper's example

Previously the example evaluated `config.stylix.image`, which should not
be done by the docs.

Since `mkEnableWallpaper` already assumes `autoEnable` is static when it
evaluates `defaultText`, we can do the same for `example`.

stylix/target.nix

@@ -113,7 +113,11 @@
               lib.literalExpression "config.stylix.image != null"
             else
               false;
-          example = config.stylix.image == null;
+          example =
+            if autoEnable then
+              lib.literalExpression "config.stylix.image == null"
+            else
+              true;
         };
 
       mkEnableIf =