Code in chapter headers can result in invalid html elements in ToC

In the Rust Book chapter 15.1 it has the header

```markdown
## Using `Box<T>` to Point to Data on the Heap
```

This is rendered into the table of contents as

```html
<a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a>
```

Resulting in the `<T>` not being rendered as it is interpreted as an HTML element.