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

[Nemo157]

In the Rust Book chapter 15.1 it has the header

## Using `Box<T>` to Point to Data on the Heap

This is rendered into the table of contents as

<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.

[ehuss]

This should be fixed via #1376.

[dtolnay]

Confirmed fixed.