Fix banner URL in README
Recent changes moved the banner away from the images directory and the README doesn't render it anymore.
<p align="center">
<a href="https://github.com/urllib3/urllib3">
- <img src="./docs/images/banner.svg" width="60%" alt="urllib3" />
+ <img src="./docs/_static/banner.svg" width="60%" alt="urllib3" />
</a>
</p>
<p align="center">
