Fix the check on fillform

The check on the fillform example test worked but is not quite correct.
Check actually expects a function that returns a boolean, as is is
documented.

examples/fillform.js

@@ -39,7 +39,7 @@ export default async function() {
       page.locator('input[type="submit"]').click(),
     ]);
     check(page, {
-      'header': page.locator('h2').textContent() == 'Welcome, admin!',
+      'header': page => page.locator('h2').textContent() == 'Welcome, admin!',
     });
 
     // Check whether we receive cookies from the logged site.