docs(forms): close the unterminated code fences in the compatForm docs

Two of the three `compatForm` overloads open a fenced code block in their
`@example` and never close it. The fence then runs to the end of the tag, so the
API renderer emits a plain `<pre><code>` block for those two instead of a
highlighted one.

The result is that on the compatForm API page the first overload's example is
syntax highlighted while the other two, which contain identical code, render as
flat unstyled text.

Rebuilding //adev/src/assets:content with and without this change alters 1 of
1565 pages. That page goes from 6 highlighted and 2 plain code blocks to 8
highlighted and none plain.

packages/forms/signals/compat/src/api/compat_form.ts

@@ -75,6 +75,7 @@ export function compatForm<TModel>(model: WritableSignal<TModel>): FieldTree<TMo
  * });
  *
  * nameForm.last().value(); // lastName, not FormControl
+ * ```
  *
  * @param model A writable signal that contains the model data for the form. The resulting field
  * structure will match the shape of the model and any changes to the form data will be written to
@@ -114,6 +115,7 @@ export function compatForm<TModel>(
  * });
  *
  * nameForm.last().value(); // lastName, not FormControl
+ * ```
  *
  * @param model A writable signal that contains the model data for the form. The resulting field
  * structure will match the shape of the model and any changes to the form data will be written to