Change `nav` to contain flow content -

I'm pretty sure you should be able to do

```html
<nav>
  <ol>
    <li>Test</li>
  </ol>
</nav>
```

typed-html/src/elements.rs

@@ -238,7 +238,7 @@ declare_elements!{
         optimum: isize,
         form: Id,
     } in [FlowContent, PhrasingContent] with PhrasingContent;
-    nav in [FlowContent, SectioningContent] with PhrasingContent;
+    nav in [FlowContent, SectioningContent] with FlowContent;
     noscript in [MetadataContent, FlowContent, PhrasingContent] with Node;
     object {
         data: Uri,