docs: update logo url
<h1 align="center">
- <a href="libp2p.io"><img width="250" src="https://github.com/libp2p/libp2p/blob/master/logo/alternates/libp2p-logo-alt-2.png?raw=true" alt="libp2p hex logo" /></a>
+ <a href="libp2p.io"><img width="250" src="https://github.com/libp2p/libp2p/blob/master/logo/black-bg-2.png?raw=true" alt="libp2p hex logo" /></a>
</h1>
<h3 align="center">The Go implementation of the libp2p Networking Stack.</h3>
