LibWeb: Fix syntax for Table test

The parser fixes this by inserting the <tr> tags, but for this test it's
better to have perfect syntax - we're not testing the parser here.

Userland/Libraries/LibWeb/Tests/Pages/Table.html

@@ -8,7 +8,9 @@
                 A Caption
             </caption>
             <thead>
-                <th>Head Cell</th>
+                <tr>
+                    <th>Head Cell</th>
+                </tr>
             </thead>
             <tbody>
                 <tr>