Update solution.md

Author: Violet-Bora-Lee

1-js/99-js-misc/01-proxy/01-error-nonexisting/solution.md

@@ -19,5 +19,5 @@ function wrap(target) {
 user = wrap(user);
 
 alert(user.name); // John
-alert(user.age); // Error: Property doesn't exist
+alert(user.age); // ReferenceError: Property doesn't exist
 ```