fix: pretty-date example in docs

This allows to copy&paste the example without running in errors, and without changing current semantics. I, however, believe the example is oudated because what the core `pretty-date` component actually does is:

```html
<time @name="pretty-date" :datetime="toIso(date)">
    { pretty(date) }

    <script>
        pretty(date) {
            return new(date).toLocaleDateString(this.locale, {
                year: 'numeric',
                month: 'long',
                day: 'numeric'
            });
        }

        toIso(date) {
            return new Date(date).toISOString();
        }
    </script>
</time>
```

Author: NickSdot

packages/nuejs.org/docs/custom-components.md

@@ -271,7 +271,7 @@ Properties and methods are defined inside a `<script>` block that is a direct ch
     // Constructor method runs when the component is created
     constructor({ date, locale }) {
       if (locale) this.locale = locale;
-      this.date = date;
+      this.date = new Date(date);
     }
 
     // Method to format the date into a readable string