css: update selectors for odig package title
this is for the package index pages.
it appears that the new heading structure is something like this:
```
<header class="odoc-preamble">
<h1 id="package-angstrom">
<a href="#package-angstrom" class="anchor"></a>
Package angstrom
<span class="version">0.16.1</span>
<nav>
<a href="https://github.com/inhabitedtype/angstrom/issues">issues</a>
<a href="#package_info">more…</a>
</nav>
</h1>
```
notably, the h1 element doesn't have a .package classname anymore. the new selectors are borrowed from the odig css.