Escape < and > in rendered toc to match handlebars' escaping in <title>

[dtolnay]

The previous behavior was inconsistent with the rendering of <title> through handlebars, making it impossible to render a matching chapter name in both places.

Before:

SUMMARY.md	rendered title	rendered sidebar
[Vec<T>](vec.md)	<title>Vec</title> ❌	<a ...>Vec</a> ❌
[Vec&lt;T&gt;](vec.md)	<title>Vec&lt;T&gt;</title> ✔️	<a ...>Vec<T></a> ❌
[Vec&amp;lt;T&amp;gt;](vec.md)	<title>Vec&amp;lt;T&amp;gt;</title> ❌	<a ...>Vec&lt;T&gt;</a> ✔️

After:

SUMMARY.md	rendered title	rendered sidebar
[Vec<T>](vec.md)	<title>Vec</title> ❌	<a ...>Vec</a> ❌
[Vec&lt;T&gt;](vec.md)	<title>Vec&lt;T&gt;</title> ✔️	<a ...>Vec&lt;T&gt;</a> ✔️
[Vec&amp;lt;T&amp;gt;](vec.md)	<title>Vec&amp;lt;T&amp;gt;</title> ❌	<a ...>Vec&amp;lt;T&amp;gt;</a> ❌

[ehuss]

Thanks!