Fix the code sample from dart web get started guide (#5688)

I have discovered that the dart web [get started](https://dart.dev/web/get-started) code example does not match
the new dart:web interface. Therefore I have updated the documentation.

Fixes: There is no specific issue, but it certainly helps #5674 and
#5525

---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>

src/content/web/get-started.md

@@ -105,23 +105,28 @@ Let's customize the app you just created.
    It creates a new `LIElement` containing the specified `String`.
 
    ```dart
-   Iterable<String> thingsTodo() sync* { ... }
+   Iterable<String> thingsTodo() sync* { /* ... */ }
 
-   [!LIElement newLI(String itemText) => LIElement()..text = itemText;!]
-
-   void main() { ... }
+   [!HTMLLIElement newLI(String itemText) =>!]
+     [!(document.createElement('li') as HTMLLIElement)..text = itemText;!]
+    
+   void main() { /* ... */ }
    ```
 
-3. In the `main()` function, initialize the `output` element using
-   `thingsTodo()`:
+3. In the `main()` function, append content to the `output` element
+   using `appendChild` and the values from `thingsTodo()`:
 
    ```dart
-   Iterable<String> thingsTodo() sync* { ... }
+   Iterable<String> thingsTodo() sync* { /* ... */ }
 
-   LIElement newLI(String itemText) => LIElement()..text = itemText;
+   HTMLLIElement newLI(String itemText) =>
+     (document.createElement('li') as HTMLLIElement)..text = itemText;
 
    void main() {
-     querySelector('#output')?[!.children.addAll(thingsTodo().map(newLI));!]
+    final output = querySelector('#output');
+    [!for (final item in thingsTodo()) {!]
+      [!output?.appendChild(newLI(item));!]
+    [!}!]
    }
    ```