doc: fixed documentation error for validate function (#387)

guides/4.MODES.md

@@ -116,7 +116,7 @@ polygonMode = new TerraDrawPolygonMode({
     if (updateType === "finish" || updateType === "commit") {
       return ValidateNotSelfIntersecting(feature);
     }
-    return true
+    return { valid: true }
   }
 });
 ```