docs: Fix broken header image
<div align="center">
<a href="https://pypi.org/project/discord-py-interactions/">
- <img src="https://raw.githubusercontent.com/muqshots/discord-py-interactions/master/.github/banner_transparent.png" alt="discord-py-interactions" height="128">
+ <img src=".github/banner_transparent.png" alt="discord-py-interactions" height="128">
</a>
<h2>Your ultimate Discord interactions library for <a href="https://github.com/Rapptz/discord.py">discord.py</a>.</h2>
</div>
